From: Andy Lutomirski <luto@xxxxxxxxxx> This patch has been added to the 3.12 stable tree. If you have any objections, please let us know. =============== commit 810bc075f78ff2c221536eb3008eac6a492dba2d upstream. We have a tricky bug in the nested NMI code: if we see RSP pointing to the NMI stack on NMI entry from kernel mode, we assume that we are executing a nested NMI. This isn't quite true. A malicious userspace program can point RSP at the NMI stack, issue SYSCALL, and arrange for an NMI to happen while RSP is still pointing at the NMI stack. Fix it with a sneaky trick. Set DF in the region of code that the RSP check is intended to detect. IRET will clear DF atomically. ( Note: other than paravirt, there's little need for all this complexity. We could check RIP instead of RSP. ) Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx> Reviewed-by: Steven Rostedt <rostedt@xxxxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Signed-off-by: Jiri Slaby <jslaby@xxxxxxx> --- arch/x86/kernel/entry_64.S | 29 ++++++++++++++++++++++++++--- 1 file changed, 26 insertions(+), 3 deletions(-) diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S index b58cfdd0bc50..7ed99df028ca 100644 --- a/arch/x86/kernel/entry_64.S +++ b/arch/x86/kernel/entry_64.S @@ -1847,10 +1847,25 @@ ENTRY(nmi) /* * Now test if the previous stack was an NMI stack. This covers * the case where we interrupt an outer NMI after it clears - * "NMI executing" but before IRET. + * "NMI executing" but before IRET. We need to be careful, though: + * there is one case in which RSP could point to the NMI stack + * despite there being no NMI active: naughty userspace controls + * RSP at the very beginning of the SYSCALL targets. We can + * pull a fast one on naughty userspace, though: we program + * SYSCALL to mask DF, so userspace cannot cause DF to be set + * if it controls the kernel's RSP. We set DF before we clear + * "NMI executing". */ lea 6*8(%rsp), %rdx test_in_nmi rdx, 4*8(%rsp), nested_nmi, first_nmi + + /* Ah, it is within the NMI stack. */ + + testb $(X86_EFLAGS_DF >> 8), (3*8 + 1)(%rsp) + jz first_nmi /* RSP was user controlled. */ + + /* This is a nested NMI. */ + CFI_REMEMBER_STATE nested_nmi: @@ -1964,8 +1979,16 @@ nmi_restore: /* Pop the extra iret frame at once */ RESTORE_ALL 6*8 - /* Clear "NMI executing". */ - movq $0, 5*8(%rsp) + /* + * Clear "NMI executing". Set DF first so that we can easily + * distinguish the remaining code between here and IRET from + * the SYSCALL entry and exit paths. On a native kernel, we + * could just inspect RIP, but, on paravirt kernels, + * INTERRUPT_RETURN can translate into a jump into a + * hypercall page. + */ + std + movq $0, 5*8(%rsp) /* clear "NMI executing" */ /* * INTERRUPT_RETURN reads the "iret" frame and exits the NMI -- 2.6.0 -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html