Re: [PATCH] [2/11] x86: Fix alternatives and kprobes to remapwrite-protected kernel text

From: Rusty Russell
Date: Tue Jul 24 2007 - 02:06:51 EST

On Fri, 2007-07-20 at 17:32 +0200, Andi Kleen wrote:
> Reenable kprobes and alternative patching when the kernel text is write
> protected by DEBUG_RODATA
> Add a general utility function to change write protected text.
> The new function remaps the code using vmap to write it and takes
> care of CPU synchronization. It also does CLFLUSH to make icache recovery
> faster.
>
> There are some limitations on when the function can be used, see
> the comment.
>
> This is a newer version that also changes the paravirt_ops code.
> text_poke also supports multi byte patching now.

Hmm, I wrote this code originally, and would have liked to catch this
change before it went in.

It's broken on i386 w/ paravirt_ops.

Problem: lookup_address() contains paravirt_ops. So we call
paravirt_ops.patch which patches part of the instructions, then calls
nop_out to patch the end of the instructions. This calls text_poke
which calls lookup_address, which is what we've half-patched. Mainly,
we get lucky at the moment (I just hit this in lguest).

We could use a simpler nop_out at boot, or fix this a little more
robustly by making everyone do their patching via a single call to
text_poke.

Needs testing for VMI and Xen (lguest and native booted here):
===
Make patching more robust, fix paravirt issue

Commit 19d36ccdc34f5ed444f8a6af0cbfdb6790eb1177 "x86: Fix alternatives
and kprobes to remap write-protected kernel text" uses code which is
being patched for patching.

In particular, paravirt_ops does patching in two stages: first it
calls paravirt_ops.patch, then it fills any remaining instructions
with nop_out(). nop_out calls text_poke() which calls
lookup_address() which calls pgd_val() (aka paravirt_ops.pgd_val):
that call site is one of the places we patch.

If we always do patching as one single call to text_poke(), we only
need make sure we're not patching the memcpy in text_poke itself.
This means the prototype to paravirt_ops.patch needs to change, to
marshal the new code into a buffer rather than patching in place as it
does now. It also means all patching goes through text_poke(), which
is known to be safe (apply_alternatives is also changed to make a
single patch).

Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>

diff -r b81206bbf749 arch/i386/kernel/alternative.c
--- a/arch/i386/kernel/alternative.c Tue Jul 24 13:05:36 2007 +1000
+++ b/arch/i386/kernel/alternative.c Tue Jul 24 14:50:54 2007 +1000
@@ -10,6 +10,8 @@
#include <asm/pgtable.h>
#include <asm/mce.h>
#include <asm/nmi.h>
+
+#define MAX_PATCH_LEN 128

#ifdef CONFIG_HOTPLUG_CPU
static int smp_alt_once;
@@ -148,7 +150,8 @@ static unsigned char** find_nop_table(vo

#endif /* CONFIG_X86_64 */

-static void nop_out(void *insns, unsigned int len)
+/* Use this to add nops to a buffer, then text_poke the whole buffer. */
+static void add_nops(void *insns, unsigned int len)
{
unsigned char **noptable = find_nop_table();

@@ -156,7 +159,7 @@ static void nop_out(void *insns, unsigne
unsigned int noplen = len;
if (noplen > ASM_NOP_MAX)
noplen = ASM_NOP_MAX;
- text_poke(insns, noptable[noplen], noplen);
+ memcpy(insns, noptable[noplen], noplen);
insns += noplen;
len -= noplen;
}
@@ -174,15 +177,14 @@ void apply_alternatives(struct alt_instr
void apply_alternatives(struct alt_instr *start, struct alt_instr *end)
{
struct alt_instr *a;
- u8 *instr;
- int diff;
+ char insnbuf[MAX_PATCH_LEN];

DPRINTK("%s: alt table %p -> %p\n", __FUNCTION__, start, end);
for (a = start; a < end; a++) {
BUG_ON(a->replacementlen > a->instrlen);
+ BUG_ON(a->instrlen > sizeof(insnbuf));
if (!boot_cpu_has(a->cpuid))
continue;
- instr = a->instr;
#ifdef CONFIG_X86_64
/* vsyscall code is not mapped yet. resolve it manually. */
if (instr >= (u8 *)VSYSCALL_START && instr < (u8*)VSYSCALL_END) {
@@ -191,9 +193,10 @@ void apply_alternatives(struct alt_instr
__FUNCTION__, a->instr, instr);
}
#endif
- memcpy(instr, a->replacement, a->replacementlen);
- diff = a->instrlen - a->replacementlen;
- nop_out(instr + a->replacementlen, diff);
+ memcpy(insnbuf, a->replacement, a->replacementlen);
+ add_nops(insnbuf + a->replacementlen,
+ a->instrlen - a->replacementlen);
+ text_poke(a->instr, insnbuf, a->instrlen);
}
}

@@ -215,16 +218,18 @@ static void alternatives_smp_unlock(u8 *
static void alternatives_smp_unlock(u8 **start, u8 **end, u8 *text, u8 *text_end)
{
u8 **ptr;
+ char insn[1];

if (noreplace_smp)
return;

+ add_nops(insn, 1);
for (ptr = start; ptr < end; ptr++) {
if (*ptr < text)
continue;
if (*ptr > text_end)
continue;
- nop_out(*ptr, 1);
+ text_poke(*ptr, insn, 1);
};
}

@@ -351,6 +356,7 @@ void apply_paravirt(struct paravirt_patc
struct paravirt_patch_site *end)
{
struct paravirt_patch_site *p;
+ char insnbuf[MAX_PATCH_LEN];

if (noreplace_paravirt)
return;
@@ -358,13 +364,15 @@ void apply_paravirt(struct paravirt_patc
for (p = start; p < end; p++) {
unsigned int used;

- used = paravirt_ops.patch(p->instrtype, p->clobbers, p->instr,
- p->len);
+ BUG_ON(p->len > MAX_PATCH_LEN);
+ used = paravirt_ops.patch(p->instrtype, p->clobbers, insnbuf,
+ (unsigned long)p->instr, p->len);

BUG_ON(used > p->len);

/* Pad the rest with nops */
- nop_out(p->instr + used, p->len - used);
+ add_nops(insnbuf + used, p->len - used);
+ text_poke(p->instr, insnbuf, p->len);
}
}
extern struct paravirt_patch_site __start_parainstructions[],
diff -r b81206bbf749 arch/i386/kernel/paravirt.c
--- a/arch/i386/kernel/paravirt.c Tue Jul 24 13:05:36 2007 +1000
+++ b/arch/i386/kernel/paravirt.c Tue Jul 24 15:13:32 2007 +1000
@@ -69,7 +69,8 @@ DEF_NATIVE(read_tsc, "rdtsc");

DEF_NATIVE(ud2a, "ud2a");

-static unsigned native_patch(u8 type, u16 clobbers, void *insns, unsigned len)
+static unsigned native_patch(u8 type, u16 clobbers, void *ibuf,
+ unsigned long addr, unsigned len)
{
const unsigned char *start, *end;
unsigned ret;
@@ -90,7 +91,7 @@ static unsigned native_patch(u8 type, u1
#undef SITE

patch_site:
- ret = paravirt_patch_insns(insns, len, start, end);
+ ret = paravirt_patch_insns(ibuf, len, start, end);
break;

case PARAVIRT_PATCH(make_pgd):
@@ -107,7 +108,7 @@ static unsigned native_patch(u8 type, u1
break;

default:
- ret = paravirt_patch_default(type, clobbers, insns, len);
+ ret = paravirt_patch_default(type, clobbers, ibuf, addr, len);
break;
}

@@ -129,68 +130,67 @@ struct branch {
u32 delta;
} __attribute__((packed));

-unsigned paravirt_patch_call(void *target, u16 tgt_clobbers,
- void *site, u16 site_clobbers,
+unsigned paravirt_patch_call(void *insnbuf,
+ const void *target, u16 tgt_clobbers,
+ unsigned long addr, u16 site_clobbers,
unsigned len)
{
- unsigned char *call = site;
- unsigned long delta = (unsigned long)target - (unsigned long)(call+5);
- struct branch b;
+ struct branch *b = insnbuf;
+ unsigned long delta = (unsigned long)target - (addr+5);

if (tgt_clobbers & ~site_clobbers)
return len; /* target would clobber too much for this site */
if (len < 5)
return len; /* call too long for patch site */

- b.opcode = 0xe8; /* call */
- b.delta = delta;
- BUILD_BUG_ON(sizeof(b) != 5);
- text_poke(call, (unsigned char *)&b, 5);
+ b->opcode = 0xe8; /* call */
+ b->delta = delta;
+ BUILD_BUG_ON(sizeof(*b) != 5);

return 5;
}

-unsigned paravirt_patch_jmp(void *target, void *site, unsigned len)
-{
- unsigned char *jmp = site;
- unsigned long delta = (unsigned long)target - (unsigned long)(jmp+5);
- struct branch b;
+unsigned paravirt_patch_jmp(const void *target, void *insnbuf,
+ unsigned long addr, unsigned len)
+{
+ struct branch *b = insnbuf;
+ unsigned long delta = (unsigned long)target - (addr+5);

if (len < 5)
return len; /* call too long for patch site */

- b.opcode = 0xe9; /* jmp */
- b.delta = delta;
- text_poke(jmp, (unsigned char *)&b, 5);
+ b->opcode = 0xe9; /* jmp */
+ b->delta = delta;

return 5;
}

-unsigned paravirt_patch_default(u8 type, u16 clobbers, void *site, unsigned len)
+unsigned paravirt_patch_default(u8 type, u16 clobbers, void *insnbuf,
+ unsigned long addr, unsigned len)
{
void *opfunc = *((void **)&paravirt_ops + type);
unsigned ret;

if (opfunc == NULL)
/* If there's no function, patch it with a ud2a (BUG) */
- ret = paravirt_patch_insns(site, len, start_ud2a, end_ud2a);
+ ret = paravirt_patch_insns(insnbuf, len, start_ud2a, end_ud2a);
else if (opfunc == paravirt_nop)
/* If the operation is a nop, then nop the callsite */
ret = paravirt_patch_nop();
else if (type == PARAVIRT_PATCH(iret) ||
type == PARAVIRT_PATCH(irq_enable_sysexit))
/* If operation requires a jmp, then jmp */
- ret = paravirt_patch_jmp(opfunc, site, len);
+ ret = paravirt_patch_jmp(opfunc, insnbuf, addr, len);
else
/* Otherwise call the function; assume target could
clobber any caller-save reg */
- ret = paravirt_patch_call(opfunc, CLBR_ANY,
- site, clobbers, len);
+ ret = paravirt_patch_call(insnbuf, opfunc, CLBR_ANY,
+ addr, clobbers, len);

return ret;
}

-unsigned paravirt_patch_insns(void *site, unsigned len,
+unsigned paravirt_patch_insns(void *insnbuf, unsigned len,
const char *start, const char *end)
{
unsigned insn_len = end - start;
@@ -198,7 +198,7 @@ unsigned paravirt_patch_insns(void *site
if (insn_len > len || start == NULL)
insn_len = len;
else
- memcpy(site, start, insn_len);
+ memcpy(insnbuf, start, insn_len);

return insn_len;
}
diff -r b81206bbf749 arch/i386/kernel/vmi.c
--- a/arch/i386/kernel/vmi.c Tue Jul 24 13:05:36 2007 +1000
+++ b/arch/i386/kernel/vmi.c Tue Jul 24 15:12:42 2007 +1000
@@ -87,12 +87,14 @@ struct vmi_timer_ops vmi_timer_ops;
#define IRQ_PATCH_INT_MASK 0
#define IRQ_PATCH_DISABLE 5

-static inline void patch_offset(unsigned char *eip, unsigned char *dest)
-{
- *(unsigned long *)(eip+1) = dest-eip-5;
-}
-
-static unsigned patch_internal(int call, unsigned len, void *insns)
+static inline void patch_offset(void *insnbuf,
+ unsigned long eip, unsigned long dest)
+{
+ *(unsigned long *)(insnbuf+1) = dest-eip-5;
+}
+
+static unsigned patch_internal(int call, unsigned len, void *insnbuf,
+ unsigned long eip)
{
u64 reloc;
struct vmi_relocation_info *const rel = (struct vmi_relocation_info *)&reloc;
@@ -100,14 +102,14 @@ static unsigned patch_internal(int call,
switch(rel->type) {
case VMI_RELOCATION_CALL_REL:
BUG_ON(len < 5);
- *(char *)insns = MNEM_CALL;
- patch_offset(insns, rel->eip);
+ *(char *)insnbuf = MNEM_CALL;
+ patch_offset(insnbuf, eip, (unsigned long)rel->eip);
return 5;

case VMI_RELOCATION_JUMP_REL:
BUG_ON(len < 5);
- *(char *)insns = MNEM_JMP;
- patch_offset(insns, rel->eip);
+ *(char *)insnbuf = MNEM_JMP;
+ patch_offset(insnbuf, eip, (unsigned long)rel->eip);
return 5;

case VMI_RELOCATION_NOP:
@@ -128,21 +130,26 @@ static unsigned patch_internal(int call,
* Apply patch if appropriate, return length of new instruction
* sequence. The callee does nop padding for us.
*/
-static unsigned vmi_patch(u8 type, u16 clobbers, void *insns, unsigned len)
+static unsigned vmi_patch(u8 type, u16 clobbers, void *insns,
+ unsigned long eip, unsigned len)
{
switch (type) {
case PARAVIRT_PATCH(irq_disable):
- return patch_internal(VMI_CALL_DisableInterrupts, len, insns);
+ return patch_internal(VMI_CALL_DisableInterrupts, len,
+ insns, eip);
case PARAVIRT_PATCH(irq_enable):
- return patch_internal(VMI_CALL_EnableInterrupts, len, insns);
+ return patch_internal(VMI_CALL_EnableInterrupts, len,
+ insns, eip);
case PARAVIRT_PATCH(restore_fl):
- return patch_internal(VMI_CALL_SetInterruptMask, len, insns);
+ return patch_internal(VMI_CALL_SetInterruptMask, len,
+ insns, eip);
case PARAVIRT_PATCH(save_fl):
- return patch_internal(VMI_CALL_GetInterruptMask, len, insns);
+ return patch_internal(VMI_CALL_GetInterruptMask, len,
+ insns, eip);
case PARAVIRT_PATCH(iret):
- return patch_internal(VMI_CALL_IRET, len, insns);
+ return patch_internal(VMI_CALL_IRET, len, insns, eip);
case PARAVIRT_PATCH(irq_enable_sysexit):
- return patch_internal(VMI_CALL_SYSEXIT, len, insns);
+ return patch_internal(VMI_CALL_SYSEXIT, len, insns, eip);
default:
break;
}
diff -r b81206bbf749 arch/i386/xen/enlighten.c
--- a/arch/i386/xen/enlighten.c Tue Jul 24 13:05:36 2007 +1000
+++ b/arch/i386/xen/enlighten.c Tue Jul 24 15:10:37 2007 +1000
@@ -842,7 +842,8 @@ void __init xen_setup_vcpu_info_placemen
}
}

-static unsigned xen_patch(u8 type, u16 clobbers, void *insns, unsigned len)
+static unsigned xen_patch(u8 type, u16 clobbers, void *insnbuf,
+ unsigned long addr, unsigned len)
{
char *start, *end, *reloc;
unsigned ret;
@@ -869,7 +870,7 @@ static unsigned xen_patch(u8 type, u16 c
if (start == NULL || (end-start) > len)
goto default_patch;

- ret = paravirt_patch_insns(insns, len, start, end);
+ ret = paravirt_patch_insns(insnbuf, len, start, end);

/* Note: because reloc is assigned from something that
appears to be an array, gcc assumes it's non-null,
@@ -877,8 +878,8 @@ static unsigned xen_patch(u8 type, u16 c
end. */
if (reloc > start && reloc < end) {
int reloc_off = reloc - start;
- long *relocp = (long *)(insns + reloc_off);
- long delta = start - (char *)insns;
+ long *relocp = (long *)(insnbuf + reloc_off);
+ long delta = start - (char *)addr;

*relocp += delta;
}
@@ -886,7 +887,8 @@ static unsigned xen_patch(u8 type, u16 c

default_patch:
default:
- ret = paravirt_patch_default(type, clobbers, insns, len);
+ ret = paravirt_patch_default(type, clobbers, insnbuf,
+ addr, len);
break;
}

diff -r b81206bbf749 drivers/lguest/lguest.c
--- a/drivers/lguest/lguest.c Tue Jul 24 13:05:36 2007 +1000
+++ b/drivers/lguest/lguest.c Tue Jul 24 15:08:20 2007 +1000
@@ -521,21 +521,22 @@ static const struct lguest_insns
[PARAVIRT_PATCH(restore_fl)] = { lgstart_popf, lgend_popf },
[PARAVIRT_PATCH(save_fl)] = { lgstart_pushf, lgend_pushf },
};
-static unsigned lguest_patch(u8 type, u16 clobber, void *insns, unsigned len)
+static unsigned lguest_patch(u8 type, u16 clobber, void *ibuf,
+ unsigned long addr, unsigned len)
{
unsigned int insn_len;

/* Don't touch it if we don't have a replacement */
if (type >= ARRAY_SIZE(lguest_insns) || !lguest_insns[type].start)
- return paravirt_patch_default(type, clobber, insns, len);
+ return paravirt_patch_default(type, clobber, ibuf, addr, len);

insn_len = lguest_insns[type].end - lguest_insns[type].start;

/* Similarly if we can't fit replacement. */
if (len < insn_len)
- return paravirt_patch_default(type, clobber, insns, len);
-
- memcpy(insns, lguest_insns[type].start, insn_len);
+ return paravirt_patch_default(type, clobber, ibuf, addr, len);
+
+ memcpy(ibuf, lguest_insns[type].start, insn_len);
return insn_len;
}

diff -r b81206bbf749 include/asm-i386/paravirt.h
--- a/include/asm-i386/paravirt.h Tue Jul 24 13:05:36 2007 +1000
+++ b/include/asm-i386/paravirt.h Tue Jul 24 14:57:44 2007 +1000
@@ -47,7 +47,8 @@ struct paravirt_ops
* The patch function should return the number of bytes of code
* generated, as we nop pad the rest in generic code.
*/
- unsigned (*patch)(u8 type, u16 clobber, void *firstinsn, unsigned len);
+ unsigned (*patch)(u8 type, u16 clobber, void *insnbuf,
+ unsigned long addr, unsigned len);

/* Basic arch-specific setup */
void (*arch_setup)(void);
@@ -253,13 +254,16 @@ extern struct paravirt_ops paravirt_ops;

unsigned paravirt_patch_nop(void);
unsigned paravirt_patch_ignore(unsigned len);
-unsigned paravirt_patch_call(void *target, u16 tgt_clobbers,
- void *site, u16 site_clobbers,
+unsigned paravirt_patch_call(void *insnbuf,
+ const void *target, u16 tgt_clobbers,
+ unsigned long addr, u16 site_clobbers,
unsigned len);
-unsigned paravirt_patch_jmp(void *target, void *site, unsigned len);
-unsigned paravirt_patch_default(u8 type, u16 clobbers, void *site, unsigned len);
-
-unsigned paravirt_patch_insns(void *site, unsigned len,
+unsigned paravirt_patch_jmp(const void *target, void *insnbuf,
+ unsigned long addr, unsigned len);
+unsigned paravirt_patch_default(u8 type, u16 clobbers, void *insnbuf,
+ unsigned long addr, unsigned len);
+
+unsigned paravirt_patch_insns(void *insnbuf, unsigned len,
const char *start, const char *end);

int paravirt_disable_iospace(void);



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