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x86/entry/64: Add missing irqflags tracing to native_load_gs_index()

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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@kernel.org>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/e1aeb0e6ba8dd430ec36c8a35e63b429698b4132.1511411918.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2017-11-22 20:39:16 -08:00 committed by Ingo Molnar
parent f68d62a567
commit ca37e57bbe
1 changed files with 8 additions and 2 deletions
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10
arch/x86/entry/entry_64.S
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@ -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
@ -943,11 +947,13 @@ 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