Re: [Patch V3] x86/ldt: correct fpu emulation access to ldt

From: Juergen Gross
Date: Thu Aug 06 2015 - 12:01:00 EST


On 08/06/2015 05:15 PM, Andy Lutomirski wrote:
On Thu, Aug 6, 2015 at 2:21 AM, Juergen Gross <jgross@xxxxxxxx> wrote:
Commit 37868fe113ff ("x86/ldt: Make modify_ldt synchronous") introduced
a new struct ldt_struct anchored at mm->context.ldt.

Adapt the x86 fpu emulation to use that new structure.

Cc: <stable@xxxxxxxxxxxxxxx> # 37868fe113ff: x86/ldt: Make modify_ldt synchronous
Cc: <stable@xxxxxxxxxxxxxxx> # a5b9e5a2f14f: x86/ldt: Make modify_ldt optional
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
arch/x86/math-emu/fpu_entry.c | 3 +--
arch/x86/math-emu/fpu_system.h | 21 ++++++++++++++++++---
arch/x86/math-emu/get_address.c | 3 +--
3 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/arch/x86/math-emu/fpu_entry.c b/arch/x86/math-emu/fpu_entry.c
index f37e84a..203318a 100644
--- a/arch/x86/math-emu/fpu_entry.c
+++ b/arch/x86/math-emu/fpu_entry.c
@@ -29,7 +29,6 @@

#include <asm/uaccess.h>
#include <asm/traps.h>
-#include <asm/desc.h>
#include <asm/user.h>
#include <asm/fpu/internal.h>

@@ -181,7 +180,7 @@ void math_emulate(struct math_emu_info *info)
math_abort(FPU_info, SIGILL);
}

- code_descriptor = LDT_DESCRIPTOR(FPU_CS);
+ code_descriptor = *FPU_get_ldt_descriptor(FPU_CS);
if (SEG_D_SIZE(code_descriptor)) {
/* The above test may be wrong, the book is not clear */
/* Segmented 32 bit protected mode */
diff --git a/arch/x86/math-emu/fpu_system.h b/arch/x86/math-emu/fpu_system.h
index 9ccecb6..d4a49d7 100644
--- a/arch/x86/math-emu/fpu_system.h
+++ b/arch/x86/math-emu/fpu_system.h
@@ -16,9 +16,24 @@
#include <linux/kernel.h>
#include <linux/mm.h>

-/* s is always from a cpu register, and the cpu does bounds checking
- * during register load --> no further bounds checks needed */
-#define LDT_DESCRIPTOR(s) (((struct desc_struct *)current->mm->context.ldt)[(s) >> 3])
+#include <asm/desc.h>
+#include <asm/mmu_context.h>
+
+static inline struct desc_struct *FPU_get_ldt_descriptor(unsigned seg)
+{
+ static struct desc_struct zero_desc;
+ struct desc_struct *ret = &zero_desc;
+
+#ifdef CONFIG_MODIFY_LDT_SYSCALL
+ seg >>= 3;
+ mutex_lock(&current->mm->context.lock);
+ if (current->mm->context.ldt && seg < current->mm->context.ldt->size)
+ ret = current->mm->context.ldt->entries + seg;
+ mutex_unlock(&current->mm->context.lock);
+#endif

Is there a good reason to return a pointer instead of returning struct
desc_struct directly? I think that, if you return a pointer, the
locking is still wrong. context.ldt can change at any point during
which IRQs are enabled (unless you hold the mutex), so I don't think
the mutex is sufficient -- the pointer can become invalid even after
this function returns.

Aah, of course. Sorry about that.

I just wanted to avoid returning a 8 byte structure on 32 bit. I'll send
V4...


Thanks,

Juergen

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