4.9-stable review patch. If anyone has any objections, please let me know.
------------------
From: Andy Lutomirski <luto@xxxxxxxxxx>
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@xxxxxxxxxx>
Cc: Borislav Petkov <bpetkov@xxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/e1aeb0e6ba8dd430ec36c8a35e63b429698b4132.1511411918.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
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
@@ -54,15 +54,19 @@ ENTRY(native_usergs_sysret64)
ENDPROC(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
@@ -868,11 +872,13 @@ idtentry simd_coprocessor_error do_simd
ENTRY(native_load_gs_index)
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
ret
END(native_load_gs_index)