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x86: save/restore eflags in context switch

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(And reset it on new thread creation)

It turns out that eflags is important to save and restore not just
because of iopl, but due to the magic bits like the NT bit, which we
don't want leaking between different threads.

Tested-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Linus Torvalds 2006-09-18 16:20:40 -07:00
parent 1d19f176a2
commit 47a5c6fa0e
2 changed files with 11 additions and 1 deletions
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4
arch/i386/kernel/entry.S
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@ -209,6 +209,10 @@ ENTRY(ret_from_fork)
GET_THREAD_INFO(%ebp)
popl %eax
CFI_ADJUST_CFA_OFFSET -4
pushl $0x0202 # Reset kernel eflags
CFI_ADJUST_CFA_OFFSET 4
popfl
CFI_ADJUST_CFA_OFFSET -4
jmp syscall_exit
CFI_ENDPROC

8
include/asm-i386/system.h
View file

@ -11,9 +11,14 @@
struct task_struct; /* one of the stranger aspects of C forward declarations.. */
extern struct task_struct * FASTCALL(__switch_to(struct task_struct *prev, struct task_struct *next));
/*
* Saving eflags is important. It switches not only IOPL between tasks,
* it also protects other tasks from NT leaking through sysenter etc.
*/
#define switch_to(prev,next,last) do { \
unsigned long esi,edi; \
asm volatile("pushl %%ebp\n\t" \
asm volatile("pushfl\n\t" /* Save flags */ \
"pushl %%ebp\n\t" \
"movl %%esp,%0\n\t" /* save ESP */ \
"movl %5,%%esp\n\t" /* restore ESP */ \
"movl $1f,%1\n\t" /* save EIP */ \
@ -21,6 +26,7 @@ extern struct task_struct * FASTCALL(__switch_to(struct task_struct *prev, struc
"jmp __switch_to\n" \
"1:\t" \
"popl %%ebp\n\t" \
"popfl" \
:"=m" (prev->thread.esp),"=m" (prev->thread.eip), \
"=a" (last),"=S" (esi),"=D" (edi) \
:"m" (next->thread.esp),"m" (next->thread.eip), \