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x86: Don't clobber top of pt_regs in nested NMI

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The nested NMI modifies the place (instruction, flags and stack)
that the first NMI will iret to.  However, the copy of registers
modified is exactly the one that is the part of pt_regs in
the first NMI.  This can change the behaviour of the first NMI.

In particular, Google's arch_trigger_all_cpu_backtrace handler
also prints regions of memory surrounding addresses appearing in
registers.  This results in handled exceptions, after which nested NMIs
start coming in.  These nested NMIs change the value of registers
in pt_regs.  This can cause the original NMI handler to produce
incorrect output.

We solve this problem by interchanging the position of the preserved
copy of the iret registers ("saved") and the copy subject to being
trampled by nested NMI ("copied").

Link: http://lkml.kernel.org/r/20121002002919.27236.14388.stgit@dungbeetle.mtv.corp.google.com

Signed-off-by: Salman Qazi <sqazi@google.com>
[ Added a needed CFI_ADJUST_CFA_OFFSET ]
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
This commit is contained in:
Salman Qazi 2012-10-01 17:29:25 -07:00 committed by Steven Rostedt
parent 269833bd5a
commit 28696f434f
1 changed files with 27 additions and 14 deletions
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41
arch/x86/kernel/entry_64.S
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@ -1699,9 +1699,10 @@ nested_nmi:
1:
/* Set up the interrupted NMIs stack to jump to repeat_nmi */
leaq -6*8(%rsp), %rdx
leaq -1*8(%rsp), %rdx
movq %rdx, %rsp
CFI_ADJUST_CFA_OFFSET 6*8
CFI_ADJUST_CFA_OFFSET 1*8
leaq -10*8(%rsp), %rdx
pushq_cfi $__KERNEL_DS
pushq_cfi %rdx
pushfq_cfi
@ -1709,8 +1710,8 @@ nested_nmi:
pushq_cfi $repeat_nmi
/* Put stack back */
addq $(11*8), %rsp
CFI_ADJUST_CFA_OFFSET -11*8
addq $(6*8), %rsp
CFI_ADJUST_CFA_OFFSET -6*8
nested_nmi_out:
popq_cfi %rdx
@ -1736,18 +1737,18 @@ first_nmi:
* +-------------------------+
* | NMI executing variable |
* +-------------------------+
* | Saved SS |
* | Saved Return RSP |
* | Saved RFLAGS |
* | Saved CS |
* | Saved RIP |
* +-------------------------+
* | copied SS |
* | copied Return RSP |
* | copied RFLAGS |
* | copied CS |
* | copied RIP |
* +-------------------------+
* | Saved SS |
* | Saved Return RSP |
* | Saved RFLAGS |
* | Saved CS |
* | Saved RIP |
* +-------------------------+
* | pt_regs |
* +-------------------------+
*
@ -1763,9 +1764,14 @@ first_nmi:
/* Set the NMI executing variable on the stack. */
pushq_cfi $1
/*
* Leave room for the "copied" frame
*/
subq $(5*8), %rsp
/* Copy the stack frame to the Saved frame */
.rept 5
pushq_cfi 6*8(%rsp)
pushq_cfi 11*8(%rsp)
.endr
CFI_DEF_CFA_OFFSET SS+8-RIP
@ -1786,12 +1792,15 @@ repeat_nmi:
* is benign for the non-repeat case, where 1 was pushed just above
* to this very stack slot).
*/
movq $1, 5*8(%rsp)
movq $1, 10*8(%rsp)
/* Make another copy, this one may be modified by nested NMIs */
addq $(10*8), %rsp
CFI_ADJUST_CFA_OFFSET -10*8
.rept 5
pushq_cfi 4*8(%rsp)
pushq_cfi -6*8(%rsp)
.endr
subq $(5*8), %rsp
CFI_DEF_CFA_OFFSET SS+8-RIP
end_repeat_nmi:
@ -1842,8 +1851,12 @@ nmi_swapgs:
SWAPGS_UNSAFE_STACK
nmi_restore:
RESTORE_ALL 8
/* Pop the extra iret frame */
addq $(5*8), %rsp
/* Clear the NMI executing stack variable */
movq $0, 10*8(%rsp)
movq $0, 5*8(%rsp)
jmp irq_return
CFI_ENDPROC
END(nmi)