[patch V2 14/16] x86/iopl: Restrict iopl() permission scope

From: Thomas Gleixner
Date: Mon Nov 11 2019 - 17:36:08 EST

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

The access to the full I/O port range can be also provided by the TSS I/O
bitmap, but that would require to copy 8k of data on scheduling in the
task. As shown with the sched out optimization TSS.io_bitmap_base can be
used to switch the incoming task to a preallocated I/O bitmap which has all
bits zero, i.e. allows access to all I/O ports.

Implementing this allows to provide an iopl() emulation mode which restricts
the IOPL level 3 permissions to I/O port access but removes the STI/CLI
permission which is coming with the hardware IOPL mechansim.

Provide a config option to switch IOPL to emulation mode, make it the
default and while at it also provide an option to disable IOPL completely.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---

V2: Fixed the 32bit build fail by increasing the cpu entry area size
Move the TSS update out of the iopl() emulation code.
---
arch/x86/Kconfig | 32 ++++++++++++++
arch/x86/include/asm/pgtable_32_types.h | 2
arch/x86/include/asm/processor.h | 20 ++++++++-
arch/x86/kernel/ioport.c | 70 +++++++++++++++++++++++---------
arch/x86/kernel/process.c | 29 +++++++++----
5 files changed, 122 insertions(+), 31 deletions(-)

--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1254,6 +1254,38 @@ config X86_VSYSCALL_EMULATION
Disabling this option saves about 7K of kernel size and
possibly 4K of additional runtime pagetable memory.

+choice
+ prompt "IOPL"
+ default X86_IOPL_EMULATION
+
+config X86_IOPL_EMULATION
+ bool "IOPL Emulation"
+ ---help---
+ Legacy IOPL support is an overbroad mechanism which allows user
+ space aside of accessing all 65536 I/O ports also to disable
+ interrupts. To gain this access the caller needs CAP_SYS_RAWIO
+ capabilities and permission from eventually active security
+ modules.
+
+ The emulation restricts the functionality of the syscall to
+ only allowing the full range I/O port access, but prevents the
+ ability to disable interrupts from user space.
+
+config X86_IOPL_LEGACY
+ bool "IOPL Legacy"
+ ---help---
+ Allow the full IOPL permissions, i.e. user space access to all
+ 65536 I/O ports and also the ability to disable interrupts, which
+ is overbroad and can result in system lockups.
+
+config X86_IOPL_NONE
+ bool "IOPL None"
+ ---help---
+ Disable the IOPL permission syscall. That's the safest option as
+ no sane application should depend on this functionality.
+
+endchoice
+
config TOSHIBA
tristate "Toshiba Laptop support"
depends on X86_32
--- a/arch/x86/include/asm/pgtable_32_types.h
+++ b/arch/x86/include/asm/pgtable_32_types.h
@@ -44,7 +44,7 @@ extern bool __vmalloc_start_set; /* set
* Define this here and validate with BUILD_BUG_ON() in pgtable_32.c
* to avoid include recursion hell
*/
-#define CPU_ENTRY_AREA_PAGES (NR_CPUS * 40)
+#define CPU_ENTRY_AREA_PAGES (NR_CPUS * 41)

#define CPU_ENTRY_AREA_BASE \
((FIXADDR_TOT_START - PAGE_SIZE * (CPU_ENTRY_AREA_PAGES + 1)) \
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -332,10 +332,14 @@ struct x86_hw_tss {
#define IO_BITMAP_BYTES (IO_BITMAP_BITS / BITS_PER_BYTE)
#define IO_BITMAP_LONGS (IO_BITMAP_BYTES / sizeof(long))

-#define IO_BITMAP_OFFSET_VALID \
+#define IO_BITMAP_OFFSET_VALID_MAP \
(offsetof(struct tss_struct, io_bitmap_bytes) - \
offsetof(struct tss_struct, x86_tss))

+#define IO_BITMAP_OFFSET_VALID_ALL \
+ (offsetof(struct tss_struct, io_bitmap_all) - \
+ offsetof(struct tss_struct, x86_tss))
+
/*
* The extra byte at the end is required by the hardware. It has all
* bits set.
@@ -344,7 +348,7 @@ struct x86_hw_tss {
* last valid byte
*/
#define __KERNEL_TSS_LIMIT \
- (IO_BITMAP_OFFSET_VALID + IO_BITMAP_BYTES + 1 - 1)
+ (IO_BITMAP_OFFSET_VALID_ALL + IO_BITMAP_BYTES + 1 - 1)

/* Base offset outside of TSS_LIMIT so unpriviledged IO causes #GP */
#define IO_BITMAP_OFFSET_INVALID (__KERNEL_TSS_LIMIT + 1)
@@ -390,6 +394,12 @@ struct tss_struct {
*/
unsigned char io_bitmap_bytes[IO_BITMAP_BYTES + 1]
__aligned(sizeof(unsigned long));
+ /*
+ * Special I/O bitmap to emulate IOPL(3). All bytes zero,
+ * except the additional byte at the end.
+ */
+ unsigned char io_bitmap_all[IO_BITMAP_BYTES + 1]
+ __aligned(sizeof(unsigned long));
} __aligned(PAGE_SIZE);

DECLARE_PER_CPU_PAGE_ALIGNED(struct tss_struct, cpu_tss_rw);
@@ -505,7 +515,13 @@ struct thread_struct {
#endif
/* IO permissions: */
struct io_bitmap *io_bitmap;
+
+ /*
+ * IOPL. Priviledge level dependent I/O permission which includes
+ * user space CLI/STI when granted.
+ */
unsigned long iopl;
+ unsigned long iopl_emul;

mm_segment_t addr_limit;

--- a/arch/x86/kernel/ioport.c
+++ b/arch/x86/kernel/ioport.c
@@ -27,15 +27,28 @@ void io_bitmap_share(struct task_struct
set_tsk_thread_flag(tsk, TIF_IO_BITMAP);
}

+static void task_update_io_bitmap(void)
+{
+ struct thread_struct *t = &current->thread;
+
+ preempt_disable();
+ if (t->iopl_emul == 3 || t->io_bitmap) {
+ /* TSS update is handled on exit to user space */
+ set_thread_flag(TIF_IO_BITMAP);
+ } else {
+ clear_thread_flag(TIF_IO_BITMAP);
+ /* Invalidate TSS */
+ tss_update_io_bitmap();
+ }
+ preempt_enable();
+}
+
void io_bitmap_exit(void)
{
struct io_bitmap *iobm = current->thread.io_bitmap;

- preempt_disable();
current->thread.io_bitmap = NULL;
- clear_thread_flag(TIF_IO_BITMAP);
- tss_update_io_bitmap();
- preempt_enable();
+ task_update_io_bitmap();
if (iobm && refcount_dec_and_test(&iobm->refcnt))
kfree(iobm);
}
@@ -151,36 +164,55 @@ SYSCALL_DEFINE3(ioperm, unsigned long, f
*/
SYSCALL_DEFINE1(iopl, unsigned int, level)
{
- struct pt_regs *regs = current_pt_regs();
struct thread_struct *t = &current->thread;
+ struct pt_regs *regs = current_pt_regs();
+ unsigned int old;

/*
* Careful: the IOPL bits in regs->flags are undefined under Xen PV
* and changing them has no effect.
*/
- unsigned int old = t->iopl >> X86_EFLAGS_IOPL_BIT;
+ if (IS_ENABLED(CONFIG_X86_IOPL_NONE))
+ return -ENOSYS;

if (level > 3)
return -EINVAL;
+
+ if (IS_ENABLED(CONFIG_X86_IOPL_EMULATION))
+ old = t->iopl_emul;
+ else
+ old = t->iopl >> X86_EFLAGS_IOPL_BIT;
+
+ /* No point in going further if nothing changes */
+ if (level == old)
+ return 0;
+
/* Trying to gain more privileges? */
if (level > old) {
if (!capable(CAP_SYS_RAWIO) ||
security_locked_down(LOCKDOWN_IOPORT))
return -EPERM;
}
- /*
- * Change the flags value on the return stack, which has been set
- * up on system-call entry. See also the fork and signal handling
- * code how this is handled.
- */
- regs->flags = (regs->flags & ~X86_EFLAGS_IOPL) |
- (level << X86_EFLAGS_IOPL_BIT);
- /* Store the new level in the thread struct */
- t->iopl = level << X86_EFLAGS_IOPL_BIT;
- /*
- * X86_32 switches immediately and XEN handles it via emulation.
- */
- set_iopl_mask(t->iopl);
+
+ if (IS_ENABLED(CONFIG_X86_IOPL_EMULATION)) {
+ t->iopl_emul = level;
+ task_update_io_bitmap();
+ } else {
+ /*
+ * Change the flags value on the return stack, which has
+ * been set up on system-call entry. See also the fork and
+ * signal handling code how this is handled.
+ */
+ regs->flags = (regs->flags & ~X86_EFLAGS_IOPL) |
+ (level << X86_EFLAGS_IOPL_BIT);
+ /* Store the new level in the thread struct */
+ t->iopl = level << X86_EFLAGS_IOPL_BIT;
+ /*
+ * X86_32 switches immediately and XEN handles it via
+ * emulation.
+ */
+ set_iopl_mask(t->iopl);
+ }

return 0;
}
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -368,22 +368,33 @@ static void tss_copy_io_bitmap(struct ts
void tss_update_io_bitmap(void)
{
struct tss_struct *tss = this_cpu_ptr(&cpu_tss_rw);
+ u16 *base = &tss->x86_tss.io_bitmap_base;

if (test_thread_flag(TIF_IO_BITMAP)) {
- struct io_bitmap *iobm = current->thread.io_bitmap;
+ struct thread_struct *t = &current->thread;

/*
- * Only copy bitmap data when the bitmap or the sequence
- * number differs. The update time is accounted to the
- * incoming task.
+ * IF IOPL emulation is enabled and the emulated I/O
+ * priviledge level is 3, switch to the 'grant all' bitmap.
*/
- if (tss->last_bitmap != iobm ||
- tss->last_sequence != iobm->sequence)
- tss_copy_io_bitmap(tss, iobm);
+ if (IS_ENABLED(CONFIG_X86_IOPL_EMULATION) &&
+ t->iopl_emul == 3) {
+ *base = IO_BITMAP_OFFSET_VALID_ALL;
+ } else {
+ struct io_bitmap *iobm = t->io_bitmap;

- /* Enable the bitmap */
- tss->x86_tss.io_bitmap_base = IO_BITMAP_OFFSET_VALID;
+ /*
+ * Only copy bitmap data when the bitmap or the
+ * sequence number differs. The update time is
+ * accounted to the incoming task.
+ */
+ if (tss->last_bitmap != iobm ||
+ tss->last_sequence != iobm->sequence)
+ tss_copy_io_bitmap(tss, iobm);

+ /* Enable the bitmap */
+ *base = IO_BITMAP_OFFSET_VALID_MAP;
+ }
/*
* Make sure that the TSS limit is covering the io bitmap.
* It might have been cut down by a VMEXIT to 0x67 which