[PATCH 4.6 019/121] MIPS: Prevent "restoration" of MSA context in non-MSA kernels

From: Greg Kroah-Hartman
Date: Sun Jun 05 2016 - 18:21:57 EST

4.6-stable review patch. If anyone has any objections, please let me know.


From: Paul Burton <paul.burton@xxxxxxxxxx>

commit 6533af4d4831c421cd9aa4dce7cfc19a3514cc09 upstream.

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'

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")
Tested-by: Aaro Koskinen <aaro.koskinen@xxxxxx>
Cc: James Hogan <james.hogan@xxxxxxxxxx>
Cc: Michal Toman <michal.toman@xxxxxxxxxx>
Cc: linux-mips@xxxxxxxxxxxxxx
Patchwork: https://patchwork.linux-mips.org/patch/13164/
Signed-off-by: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

arch/mips/kernel/signal.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

--- a/arch/mips/kernel/signal.c
+++ b/arch/mips/kernel/signal.c
@@ -195,6 +195,9 @@ static int restore_msa_extcontext(void _
unsigned int csr;
int i, err;

+ if (!config_enabled(CONFIG_CPU_HAS_MSA))
+ return SIGSYS;
if (size != sizeof(*msa))
return -EINVAL;

@@ -398,8 +401,8 @@ int protected_restore_fp_context(void __

- if (used & USED_EXTCONTEXT)
- err |= restore_extcontext(sc_to_extcontext(sc));
+ if (!err && (used & USED_EXTCONTEXT))
+ err = restore_extcontext(sc_to_extcontext(sc));

return err ?: sig;