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x86/asm/entry: Create and use a 'TOP_OF_KERNEL_STACK_PADDING' macro

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x86_32, unlike x86_64, pads the top of the kernel stack, because the
hardware stack frame formats are variable in size.

Document this padding and give it a name.

This should make no change whatsoever to the compiled kernel
image. It also doesn't fix any of the current bugs in this area.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Acked-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/02bf2f54b8dcb76a62a142b6dfe07d4ef7fc582e.1426009661.git.luto@amacapital.net
[ Fixed small details, such as a missed magic constant in entry_32.S pointed out by Denys Vlasenko. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2015-03-10 11:05:58 -07:00 committed by Ingo Molnar
parent 9a036b93a3
commit 3ee4298f44
3 changed files with 30 additions and 2 deletions
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3
arch/x86/include/asm/processor.h
View File

@ -849,7 +849,8 @@ extern unsigned long thread_saved_pc(struct task_struct *tsk);
#define task_pt_regs(task) \
({ \
struct pt_regs *__regs__; \
__regs__ = (struct pt_regs *)(KSTK_TOP(task_stack_page(task))-8); \
__regs__ = (struct pt_regs *)(KSTK_TOP(task_stack_page(task)) - \
TOP_OF_KERNEL_STACK_PADDING); \
__regs__ - 1; \
})

27
arch/x86/include/asm/thread_info.h
View File

@ -12,6 +12,33 @@
#include <asm/percpu.h>
#include <asm/types.h>
/*
* TOP_OF_KERNEL_STACK_PADDING is a number of unused bytes that we
* reserve at the top of the kernel stack. We do it because of a nasty
* 32-bit corner case. On x86_32, the hardware stack frame is
* variable-length. Except for vm86 mode, struct pt_regs assumes a
* maximum-length frame. If we enter from CPL 0, the top 8 bytes of
* pt_regs don't actually exist. Ordinarily this doesn't matter, but it
* does in at least one case:
*
* If we take an NMI early enough in SYSENTER, then we can end up with
* pt_regs that extends above sp0. On the way out, in the espfix code,
* we can read the saved SS value, but that value will be above sp0.
* Without this offset, that can result in a page fault. (We are
* careful that, in this case, the value we read doesn't matter.)
*
* In vm86 mode, the hardware frame is much longer still, but we neither
* access the extra members from NMI context, nor do we write such a
* frame at sp0 at all.
*
* x86_64 has a fixed-length stack frame.
*/
#ifdef CONFIG_X86_32
# define TOP_OF_KERNEL_STACK_PADDING 8
#else
# define TOP_OF_KERNEL_STACK_PADDING 0
#endif
/*
* low level task data that entry.S needs immediate access to
* - this struct should fit entirely inside of one cache line

2
arch/x86/kernel/entry_32.S
View File

@ -398,7 +398,7 @@ sysenter_past_esp:
* A tiny bit of offset fixup is necessary - 4*4 means the 4 words
* pushed above; +8 corresponds to copy_thread's esp0 setting.
*/
pushl_cfi ((TI_sysenter_return)-THREAD_SIZE+8+4*4)(%esp)
pushl_cfi ((TI_sysenter_return)-THREAD_SIZE+TOP_OF_KERNEL_STACK_PADDING+4*4)(%esp)
CFI_REL_OFFSET eip, 0
pushl_cfi %eax