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Date: Tue, 28 Nov 2017 11:24:23 +0100 From: Greg Kroah-Hartman <gregkh@...uxfoundation.org> To: linux-kernel@...r.kernel.org Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, stable@...r.kernel.org, Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bpetkov@...e.de>, Brian Gerst <brgerst@...il.com>, Dave Hansen <dave.hansen@...el.com>, Josh Poimboeuf <jpoimboe@...hat.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org> Subject: [PATCH 4.14 016/193] x86/entry/64: Add missing irqflags tracing to native_load_gs_index() 4.14-stable review patch. If anyone has any objections, please let me know. ------------------ From: Andy Lutomirski <luto@...nel.org> commit ca37e57bbe0cf1455ea3e84eb89ed04a132d59e1 upstream. Running this code with IRQs enabled (where dummy_lock is a spinlock): static void check_load_gs_index(void) { /* This will fail. */ load_gs_index(0xffff); spin_lock(&dummy_lock); spin_unlock(&dummy_lock); } Will generate a lockdep warning. The issue is that the actual write to %gs would cause an exception with IRQs disabled, and the exception handler would, as an inadvertent side effect, update irqflag tracing to reflect the IRQs-off status. native_load_gs_index() would then turn IRQs back on and return with irqflag tracing still thinking that IRQs were off. The dummy lock-and-unlock causes lockdep to notice the error and warn. Fix it by adding the missing tracing. Apparently nothing did this in a context where it mattered. I haven't tried to find a code path that would actually exhibit the warning if appropriately nasty user code were running. I suspect that the security impact of this bug is very, very low -- production systems don't run with lockdep enabled, and the warning is mostly harmless anyway. Found during a quick audit of the entry code to try to track down an unrelated bug that Ingo found in some still-in-development code. Signed-off-by: Andy Lutomirski <luto@...nel.org> Cc: Borislav Petkov <bpetkov@...e.de> Cc: Brian Gerst <brgerst@...il.com> Cc: Dave Hansen <dave.hansen@...el.com> Cc: Josh Poimboeuf <jpoimboe@...hat.com> Cc: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Peter Zijlstra <peterz@...radead.org> Cc: Thomas Gleixner <tglx@...utronix.de> Link: http://lkml.kernel.org/r/e1aeb0e6ba8dd430ec36c8a35e63b429698b4132.1511411918.git.luto@kernel.org Signed-off-by: Ingo Molnar <mingo@...nel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org> --- arch/x86/entry/entry_64.S | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -51,15 +51,19 @@ ENTRY(native_usergs_sysret64) END(native_usergs_sysret64) #endif /* CONFIG_PARAVIRT */ -.macro TRACE_IRQS_IRETQ +.macro TRACE_IRQS_FLAGS flags:req #ifdef CONFIG_TRACE_IRQFLAGS - bt $9, EFLAGS(%rsp) /* interrupts off? */ + bt $9, \flags /* interrupts off? */ jnc 1f TRACE_IRQS_ON 1: #endif .endm +.macro TRACE_IRQS_IRETQ + TRACE_IRQS_FLAGS EFLAGS(%rsp) +.endm + /* * When dynamic function tracer is enabled it will add a breakpoint * to all locations that it is about to modify, sync CPUs, update @@ -923,11 +927,13 @@ ENTRY(native_load_gs_index) FRAME_BEGIN pushfq DISABLE_INTERRUPTS(CLBR_ANY & ~CLBR_RDI) + TRACE_IRQS_OFF SWAPGS .Lgs_change: movl %edi, %gs 2: ALTERNATIVE "", "mfence", X86_BUG_SWAPGS_FENCE SWAPGS + TRACE_IRQS_FLAGS (%rsp) popfq FRAME_END ret
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