4.9-stable review patch. If anyone has any objections, please let me know.
------------------
From: Andy Lutomirski luto@kernel.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@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 Link: http://lkml.kernel.org/r/e1aeb0e6ba8dd430ec36c8a35e63b429698b4132.1511411918... Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.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 @@ -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)