powerpc/64s: Mask SRR0 before checking against the masked NIP

BugLink: https://bugs.launchpad.net/bugs/1959879
[ Upstream commit aee101d7b95a03078945681dd7f7ea5e4a1e7686 ]

Commit 314f6c23dd8d ("powerpc/64s: Mask NIP before checking against
SRR0") masked off the low 2 bits of the NIP value in the interrupt
stack frame in case they are non-zero and mis-compare against a SRR0
register value of a CPU which always reads back 0 from the 2 low bits
which are reserved.

This now causes the opposite problem that an implementation which does
implement those bits in SRR0 will mis-compare against the masked NIP
value in which they have been cleared. QEMU is one such implementation,
and this is allowed by the architecture.

This can be triggered by sigfuz by setting low bits of PT_NIP in the
signal context.

Fix this for now by masking the SRR0 bits as well. Cleaner is probably
to sanitise these values before putting them in registers or stack, but
this is the quick and backportable fix.

Fixes: 314f6c23dd8d ("powerpc/64s: Mask NIP before checking against SRR0")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220117134403.2995059-1-npiggin@gmail.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>

diff --git a/arch/powerpc/kernel/interrupt_64.S b/arch/powerpc/kernel/interrupt_64.S
index 4b1ff94e67eb40cac47300884c1b670eebae288b..4c6d1a8dcefed649b33ad2db3a7eb8fc404e116b 100644 (file)
--- a/arch/powerpc/kernel/interrupt_64.S
+++ b/arch/powerpc/kernel/interrupt_64.S
@@ -30,6 +30,7 @@ COMPAT_SYS_CALL_TABLE:
        .ifc \srr,srr
        mfspr   r11,SPRN_SRR0
        ld      r12,_NIP(r1)
+       clrrdi  r11,r11,2
        clrrdi  r12,r12,2
 100:   tdne    r11,r12
        EMIT_WARN_ENTRY 100b,__FILE__,__LINE__,(BUGFLAG_WARNING | BUGFLAG_ONCE)
@@ -40,6 +41,7 @@ COMPAT_SYS_CALL_TABLE:
        .else
        mfspr   r11,SPRN_HSRR0
        ld      r12,_NIP(r1)
+       clrrdi  r11,r11,2
        clrrdi  r12,r12,2
 100:   tdne    r11,r12
        EMIT_WARN_ENTRY 100b,__FILE__,__LINE__,(BUGFLAG_WARNING | BUGFLAG_ONCE)