powerpc: Fix msr check in compat_sys_swapcontext

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>
Signed-off-by: Paul Mackerras <paulus@samba.org>

1 files changed, 15 insertions, 3 deletions

diff --git a/arch/powerpc/kernel/signal_32.c b/arch/powerpc/kernel/signal_32.c
index a6a43103655e..b13abf305996 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)