Hi,
On Thu, Apr 21, 2016 at 06:04:53PM +0100, Paul Burton wrote:
> 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+
This patch is needed to build MIPS kernel with GCC 6.1.
Tested-by: Aaro Koskinen <aaro.koskinen@xxxxxx>
Thanks,
A.
> ---
>
> Changes in v2:
> - Prevent potential for malformed errno/signal from protected_restore_fp_context.
>
> arch/mips/kernel/signal.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/arch/mips/kernel/signal.c b/arch/mips/kernel/signal.c
> index bf792e2..fc7c1f0 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;
>
> @@ -398,8 +401,8 @@ int protected_restore_fp_context(void __user *sc)
> }
>
> fp_done:
> - 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;
> }
> --
> 2.8.0
>
>
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