[PATCH] MIPS: Prevent "restoration" of MSA context in non-MSA kernels

[PATCH] MIPS: Prevent "restoration" of MSA context in non-MSA kernels

[Paul Burton]

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@imgtec.com>
Reported-by: Michal Toman <michal.toman@imgtec.com>
Fixes: bf82cb30c7e5 ("MIPS: Save MSA extended context around signals")
Cc: James Hogan <james.hogan@imgtec.com>
Cc: stable <stable@vger.kernel.org> # 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

[PATCH] MIPS: Prevent "restoration" of MSA context in non-MSA kernels

[Paul Burton]

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@imgtec.com>
Reported-by: Michal Toman <michal.toman@imgtec.com>
Fixes: bf82cb30c7e5 ("MIPS: Save MSA extended context around signals")
Cc: James Hogan <james.hogan@imgtec.com>
Cc: stable <stable@vger.kernel.org> # 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

Re: [PATCH] MIPS: Prevent "restoration" of MSA context in non-MSA kernels

[Maciej W. Rozycki]

On Thu, 21 Apr 2016, Paul Burton wrote:

> 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;

 The priciple of your change looks reasonable itself to me, however its 
call site is ill-formed making it possible to return a nonsensical error 
code:

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

	return err ?: sig;
}

(`restore_extcontext' takes the result from `restore_msa_extcontext' and 
passes it on) so if an earlier call has set `err' to -EINVAL (which I take 
it is the only value expected here or we'd have a preexisting problem), 
then `protected_restore_fp_context' (which is where this code comes from) 
will return (-EINVAL | SIGSYS).

 So you need to redesign this code somehow I'm afraid, maybe just changing 
the condition to:

	if (!err && (used & USED_EXTCONTEXT))

will do (although while at it I'd double-check that `err' can really only 
be -EINVAL here).

  Maciej

Re: [PATCH] MIPS: Prevent "restoration" of MSA context in non-MSA kernels

[Maciej W. Rozycki]

On Thu, 21 Apr 2016, Paul Burton wrote:

> 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;

 The priciple of your change looks reasonable itself to me, however its 
call site is ill-formed making it possible to return a nonsensical error 
code:

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

	return err ?: sig;
}

(`restore_extcontext' takes the result from `restore_msa_extcontext' and 
passes it on) so if an earlier call has set `err' to -EINVAL (which I take 
it is the only value expected here or we'd have a preexisting problem), 
then `protected_restore_fp_context' (which is where this code comes from) 
will return (-EINVAL | SIGSYS).

 So you need to redesign this code somehow I'm afraid, maybe just changing 
the condition to:

	if (!err && (used & USED_EXTCONTEXT))

will do (although while at it I'd double-check that `err' can really only 
be -EINVAL here).

  Maciej

[PATCH v2] MIPS: Prevent "restoration" of MSA context in non-MSA kernels

[Paul Burton]

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@imgtec.com>
Reported-by: Michal Toman <michal.toman@imgtec.com>
Fixes: bf82cb30c7e5 ("MIPS: Save MSA extended context around signals")
Cc: James Hogan <james.hogan@imgtec.com>
Cc: stable <stable@vger.kernel.org> # v4.3+

---

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

[PATCH v2] MIPS: Prevent "restoration" of MSA context in non-MSA kernels

[Paul Burton]

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@imgtec.com>
Reported-by: Michal Toman <michal.toman@imgtec.com>
Fixes: bf82cb30c7e5 ("MIPS: Save MSA extended context around signals")
Cc: James Hogan <james.hogan@imgtec.com>
Cc: stable <stable@vger.kernel.org> # v4.3+

---

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

Re: [PATCH v2] MIPS: Prevent "restoration" of MSA context in non-MSA kernels

[Aaro Koskinen]

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@imgtec.com>
> Reported-by: Michal Toman <michal.toman@imgtec.com>
> Fixes: bf82cb30c7e5 ("MIPS: Save MSA extended context around signals")
> Cc: James Hogan <james.hogan@imgtec.com>
> Cc: stable <stable@vger.kernel.org> # v4.3+

This patch is needed to build MIPS kernel with GCC 6.1.

Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>

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
> 
>