[PATCH] Fix msr check in compat_sys_swapcontext

[PATCH] Fix msr check in compat_sys_swapcontext

[Andreas Schwab]

The new context may not be 16-byte aligned, so the real address of the
mcontext structure should be read from the uc_regs pointer instead of
directly using the (unaligned) uc_mcontext field.

Signed-off-by: Andreas Schwab <schwab@suse.de>

---
diff --git a/arch/powerpc/kernel/signal_32.c b/arch/powerpc/kernel/signal_32.c
index a6a4310..dc10720 100644
--- a/arch/powerpc/kernel/signal_32.c
+++ b/arch/powerpc/kernel/signal_32.c
@@ -941,9 +941,17 @@ long sys_swapcontext(struct ucontext __user *old_ctx,
 #ifdef CONFIG_PPC64
 	unsigned long new_msr = 0;
 
-	if (new_ctx &&
-	    get_user(new_msr, &new_ctx->uc_mcontext.mc_gregs[PT_MSR]))
-		return -EFAULT;
+	if (new_ctx) {
+		struct mcontext __user *mcp;
+		u32 cmcp;
+
+		/* Get pointer to the real mcontext. */
+		if (__get_user(cmcp, &new_ctx->uc_regs))
+			return -EFAULT;
+		mcp = (struct mcontext __user *)(u64)cmcp;
+		if (__get_user(new_msr, &mcp->mc_gregs[PT_MSR]))
+			return -EFAULT;
+	}
 	/*
 	 * Check that the context is not smaller than the original
 	 * size (with VMX but without VSX)

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Re: [PATCH] Fix msr check in compat_sys_swapcontext

[Paul Mackerras]

Andreas Schwab writes:

> The new context may not be 16-byte aligned, so the real address of the
> mcontext structure should be read from the uc_regs pointer instead of
> directly using the (unaligned) uc_mcontext field.

Good catch, but...

> @@ -941,9 +941,17 @@ long sys_swapcontext(struct ucontext __user *old_ctx,
>  #ifdef CONFIG_PPC64
>  	unsigned long new_msr = 0;
>  
> -	if (new_ctx &&
> -	    get_user(new_msr, &new_ctx->uc_mcontext.mc_gregs[PT_MSR]))
> -		return -EFAULT;
> +	if (new_ctx) {
> +		struct mcontext __user *mcp;
> +		u32 cmcp;
> +
> +		/* Get pointer to the real mcontext. */
> +		if (__get_user(cmcp, &new_ctx->uc_regs))

we need to use get_user, not __get_user, since we haven't done an
access_ok() check on the address.

> +			return -EFAULT;
> +		mcp = (struct mcontext __user *)(u64)cmcp;
> +		if (__get_user(new_msr, &mcp->mc_gregs[PT_MSR]))

ditto here.

Paul.

Re: [PATCH] Fix msr check in compat_sys_swapcontext

[Andreas Schwab]

Paul Mackerras <paulus@samba.org> writes:

> we need to use get_user, not __get_user, since we haven't done an
> access_ok() check on the address.

The address is always ok since its a compat pointer, see do_setcontext.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Re: [PATCH] Fix msr check in compat_sys_swapcontext

[Paul Mackerras]

Andreas Schwab writes:

> Paul Mackerras <paulus@samba.org> writes:
> 
> > we need to use get_user, not __get_user, since we haven't done an
> > access_ok() check on the address.
> 
> The address is always ok since its a compat pointer, see do_setcontext.

OK, since it's inside a CONFIG_PPC64 block.  I'll add a paragraph to
the patch description pointing that out.

Paul.

Re: [PATCH] Fix msr check in compat_sys_swapcontext

[Andreas Schwab]

Paul Mackerras <paulus@samba.org> writes:

> Andreas Schwab writes:
>
>> Paul Mackerras <paulus@samba.org> writes:
>> 
>> > we need to use get_user, not __get_user, since we haven't done an
>> > access_ok() check on the address.
>> 
>> The address is always ok since its a compat pointer, see do_setcontext.
>
> OK, since it's inside a CONFIG_PPC64 block.  I'll add a paragraph to
> the patch description pointing that out.

How about this:

diff --git a/arch/powerpc/kernel/signal_32.c b/arch/powerpc/kernel/signal_32.c
index a6a4310..b13abf3 100644
--- a/arch/powerpc/kernel/signal_32.c
+++ b/arch/powerpc/kernel/signal_32.c
@@ -941,9 +941,21 @@ long sys_swapcontext(struct ucontext __user *old_ctx,
 #ifdef CONFIG_PPC64
 	unsigned long new_msr = 0;
 
-	if (new_ctx &&
-	    get_user(new_msr, &new_ctx->uc_mcontext.mc_gregs[PT_MSR]))
-		return -EFAULT;
+	if (new_ctx) {
+		struct mcontext __user *mcp;
+		u32 cmcp;
+
+		/*
+		 * Get pointer to the real mcontext.  No need for
+		 * access_ok since we are dealing with compat
+		 * pointers.
+		 */
+		if (__get_user(cmcp, &new_ctx->uc_regs))
+			return -EFAULT;
+		mcp = (struct mcontext __user *)(u64)cmcp;
+		if (__get_user(new_msr, &mcp->mc_gregs[PT_MSR]))
+			return -EFAULT;
+	}
 	/*
 	 * Check that the context is not smaller than the original
 	 * size (with VMX but without VSX)

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."