If a kernel doesn't support MSA context (ie. CONFIG_CPU_HAS_MSA=n) then
it will only keep 64 bits per FP register in thread context, and the
calls to set_fpr64 in restore_msa_extcontext will overrun the end of the
FP register context into the FCSR & MSACSR values. GCC 6.x has become
smart enough to detect this & complain like so:
arch/mips/kernel/signal.c: In function 'protected_restore_fp_context':
./arch/mips/include/asm/processor.h:114:17: error: array subscript is above array bounds [-Werror=array-bounds]
fpr->val##width[FPR_IDX(width, idx)] = val; \
~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
./arch/mips/include/asm/processor.h:118:1: note: in expansion of macro 'BUILD_FPR_ACCESS'
BUILD_FPR_ACCESS(64)
The only way to trigger this code to run would be for a program to set
up an artificial extended MSA context structure following a sigframe &
execute sigreturn. Whilst this doesn't allow a program to write to any
state that it couldn't already, it makes little sense to allow this
"restoration" of MSA context in a system that doesn't support MSA.
Fix this by killing a program with SIGSYS if it tries something as crazy
as "restoring" fake MSA context in this way, also fixing the build error
& allowing for most of restore_msa_extcontext to be optimised out of
kernels without support for MSA.
Signed-off-by: Paul Burton <paul.burton@xxxxxxxxxx>
Reported-by: Michal Toman <michal.toman@xxxxxxxxxx>
Fixes: bf82cb30c7e5 ("MIPS: Save MSA extended context around signals")
Cc: James Hogan <james.hogan@xxxxxxxxxx>
Cc: stable <stable@xxxxxxxxxxxxxxx> # v4.3+
---
arch/mips/kernel/signal.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/arch/mips/kernel/signal.c b/arch/mips/kernel/signal.c
index bf792e2..a304b70 100644
--- a/arch/mips/kernel/signal.c
+++ b/arch/mips/kernel/signal.c
@@ -195,6 +195,9 @@ static int restore_msa_extcontext(void __user *buf, unsigned int size)
unsigned int csr;
int i, err;
+ if (!config_enabled(CONFIG_CPU_HAS_MSA))
+ return SIGSYS;
+
if (size != sizeof(*msa))
return -EINVAL;
--
2.8.0
