linux-kernel - Re: [PATCH] Fix msr check in compat_sys

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Date:	Thu, 06 Nov 2008 11:49:00 +0100
From:	Andreas Schwab <schwab@...e.de>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	linuxppc-dev@...abs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix msr check in compat_sys_swapcontext

Paul Mackerras <paulus@...ba.org> writes:

> Andreas Schwab writes:
>
>> Paul Mackerras <paulus@...ba.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@...e.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
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