[PATCH] x86_64: Fix canonical checking for segment registers in ptrace

Allowed user programs to set a non canonical segment base, which would cause
oopses in the kernel later.

Credit-to: Alexander Nyberg <alexn@dsv.su.se>

 For identifying and reporting this bug.
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/x86_64/kernel/ptrace.c

@@ -257,12 +257,12 @@ static int putreg(struct task_struct *child,
 			value &= 0xffff;
 			return 0;
 		case offsetof(struct user_regs_struct,fs_base):
-			if (!((value >> 48) == 0 || (value >> 48) == 0xffff))
+			if (value >= TASK_SIZE)
 				return -EIO;
 			child->thread.fs = value;
 			return 0;
 		case offsetof(struct user_regs_struct,gs_base):
-			if (!((value >> 48) == 0 || (value >> 48) == 0xffff))
+			if (value >= TASK_SIZE)
 				return -EIO;
 			child->thread.gs = value;
 			return 0;