From: "Steven Rostedt (Red Hat)" Andy Lutomirski reported an off by one in the NMI stack check for the nested NMI code, where if the stack pointer was one above the actual stack (stack start + STACK_SIZE) it would trigger a false positive. This is not that big of a deal because the stack pointer should never be that. Even if a stack was using the pages just above the NMI stack, it would require the stack about to overflow for this to trigger, which is a much bigger bug than this is fixing. Also, Linus Torvalds pointed out that doing two compares can be accomplish with a single compare. That is: ("reg" is top of stack we are comparing "stack" to) cmpq reg, stack jae label // note, code had one off "ja" instead of "jae" subq size, reg cmpq reg, stack jb label Is the same as: subq $1, reg subq stack, reg cmpq size, reg jae label The subq $1 was added into the leaq by doing: leaq 5*8+7(%rsp), %rdx Added more comments as well. Reported-by: Andy Lutomirski Inspired-by: Linus Torvalds Signed-off-by: Steven Rostedt (VMware) --- arch/x86/entry/entry_64.S | 21 +++++++++++---------- 1 file changed, 11 insertions(+), 10 deletions(-) diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index 3aad759aace2..1e6ca3740762 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -1355,16 +1355,17 @@ ENTRY(nmi) * if it controls the kernel's RSP. We set DF before we clear * "NMI executing". */ - lea 6*8(%rsp), %rdx - /* Compare the NMI stack (rdx) with the stack we came from (4*8(%rsp)) */ - cmpq %rdx, 4*8(%rsp) - /* If the stack pointer is above the NMI stack, this is a normal NMI */ - ja first_nmi - - subq $EXCEPTION_STKSZ, %rdx - cmpq %rdx, 4*8(%rsp) - /* If it is below the NMI stack, it is a normal NMI */ - jb first_nmi + + /* Load address of the top of this stack into rdx */ + lea 5*8+7(%rsp), %rdx + /* Subtract the return stack pointer from it */ + subq 4*8(%rsp), %rdx + /* + * If the result is greater or equal to the stack size, + * then the return stack was not on this stack. + */ + cmpq $EXCEPTION_STKSZ, %rdx + jae first_nmi /* Ah, it is within the NMI stack. */ -- 2.10.2