From fc57a7c68020dcf954428869eafd934c0ab1536f Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Sun, 20 Sep 2015 16:32:04 -0700 Subject: x86/paravirt: Replace the paravirt nop with a bona fide empty function PARAVIRT_ADJUST_EXCEPTION_FRAME generates this code (using nmi as an example, trimmed for readability): ff 15 00 00 00 00 callq *0x0(%rip) # 2796 2792: R_X86_64_PC32 pv_irq_ops+0x2c That's a call through a function pointer to regular C function that does nothing on native boots, but that function isn't protected against kprobes, isn't marked notrace, and is certainly not guaranteed to preserve any registers if the compiler is feeling perverse. This is bad news for a CLBR_NONE operation. Of course, if everything works correctly, once paravirt ops are patched, it gets nopped out, but what if we hit this code before paravirt ops are patched in? This can potentially cause breakage that is very difficult to debug. A more subtle failure is possible here, too: if _paravirt_nop uses the stack at all (even just to push RBP), it will overwrite the "NMI executing" variable if it's called in the NMI prologue. The Xen case, perhaps surprisingly, is fine, because it's already written in asm. Fix all of the cases that default to paravirt_nop (including adjust_exception_frame) with a big hammer: replace paravirt_nop with an asm function that is just a ret instruction. The Xen case may have other problems, so document them. This is part of a fix for some random crashes that Sasha saw. Reported-and-tested-by: Sasha Levin Signed-off-by: Andy Lutomirski Cc: stable@vger.kernel.org Link: http://lkml.kernel.org/r/8f5d2ba295f9d73751c33d97fda03e0495d9ade0.1442791737.git.luto@kernel.org Signed-off-by: Thomas Gleixner --- arch/x86/entry/entry_64.S | 11 +++++++++++ arch/x86/kernel/paravirt.c | 16 ++++++++++++---- 2 files changed, 23 insertions(+), 4 deletions(-) diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index d3033183ed70..404ca97c4715 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -1128,7 +1128,18 @@ END(error_exit) /* Runs on exception stack */ ENTRY(nmi) + /* + * Fix up the exception frame if we're on Xen. + * PARAVIRT_ADJUST_EXCEPTION_FRAME is guaranteed to push at most + * one value to the stack on native, so it may clobber the rdx + * scratch slot, but it won't clobber any of the important + * slots past it. + * + * Xen is a different story, because the Xen frame itself overlaps + * the "NMI executing" variable. + */ PARAVIRT_ADJUST_EXCEPTION_FRAME + /* * We allow breakpoints in NMIs. If a breakpoint occurs, then * the iretq it performs will take us out of NMI context. diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c index f68e48f5f6c2..c2130aef3f9d 100644 --- a/arch/x86/kernel/paravirt.c +++ b/arch/x86/kernel/paravirt.c @@ -41,10 +41,18 @@ #include #include -/* nop stub */ -void _paravirt_nop(void) -{ -} +/* + * nop stub, which must not clobber anything *including the stack* to + * avoid confusing the entry prologues. + */ +extern void _paravirt_nop(void); +asm (".pushsection .entry.text, \"ax\"\n" + ".global _paravirt_nop\n" + "_paravirt_nop:\n\t" + "ret\n\t" + ".size _paravirt_nop, . - _paravirt_nop\n\t" + ".type _paravirt_nop, @function\n\t" + ".popsection"); /* identity function, which can be inlined */ u32 _paravirt_ident_32(u32 x) -- cgit v1.2.3-58-ga151 From 83c133cf11fb0e68a51681447e372489f052d40e Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Sun, 20 Sep 2015 16:32:05 -0700 Subject: x86/nmi/64: Fix a paravirt stack-clobbering bug in the NMI code The NMI entry code that switches to the normal kernel stack needs to be very careful not to clobber any extra stack slots on the NMI stack. The code is fine under the assumption that SWAPGS is just a normal instruction, but that assumption isn't really true. Use SWAPGS_UNSAFE_STACK instead. This is part of a fix for some random crashes that Sasha saw. Fixes: 9b6e6a8334d5 ("x86/nmi/64: Switch stacks on userspace NMI entry") Reported-and-tested-by: Sasha Levin Signed-off-by: Andy Lutomirski Cc: stable@vger.kernel.org Link: http://lkml.kernel.org/r/974bc40edffdb5c2950a5c4977f821a446b76178.1442791737.git.luto@kernel.org Signed-off-by: Thomas Gleixner --- arch/x86/entry/entry_64.S | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index 404ca97c4715..055a01de7c8d 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -1190,9 +1190,12 @@ ENTRY(nmi) * we don't want to enable interrupts, because then we'll end * up in an awkward situation in which IRQs are on but NMIs * are off. + * + * We also must not push anything to the stack before switching + * stacks lest we corrupt the "NMI executing" variable. */ - SWAPGS + SWAPGS_UNSAFE_STACK cld movq %rsp, %rdx movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp -- cgit v1.2.3-58-ga151