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Subject[PATCH] x86/entry/64: Add missing irqflags tracing to native_load_gs_index()
Date
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.

Cc: stable@vger.kernel.org
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---

Hi Ingo-

You asked me to look for an irqflag tracing bug, so I found one :)

arch/x86/entry/entry_64.S | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index a2b30ec69497..3c288f260fdf 100644
--- 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
@@ -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
--
2.13.6
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