Patch "MIPS: Fix buffer overflow in syscall_get_arguments()" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

MIPS: Fix buffer overflow in syscall_get_arguments()

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mips-fix-buffer-overflow-in-syscall_get_arguments.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: James Hogan <[email protected]>
Date: Mon, 25 Jan 2016 20:32:03 +0000
Subject: MIPS: Fix buffer overflow in syscall_get_arguments()

From: James Hogan <[email protected]>

commit f4dce1ffd2e30fa31756876ef502ce6d2324be35 upstream.

Since commit 4c21b8fd8f14 ("MIPS: seccomp: Handle indirect system calls (o32)"), syscall_get_arguments() attempts to handle o32 indirect syscall arguments by incrementing both the start argument number and the number of arguments to fetch. However only the start argument number needs to be incremented. The number of arguments does not change, they're just shifted up by one, and in fact the output array is provided by the caller and is likely only n entries long, so reading more arguments overflows the output buffer.

In the case of seccomp, this results in it fetching 7 arguments starting at the 2nd one, which overflows the unsigned long args[6] in populate_seccomp_data(). This clobbers the $s0 register from syscall_trace_enter() which __seccomp_phase1_filter() saved onto the stack, into which syscall_trace_enter() had placed its syscall number argument. This caused Chromium to crash.

Credit goes to Milko for tracking it down as far as $s0 being clobbered.

Patchwork: https://patchwork.linux-mips.org/patch/12213/

---
 arch/mips/include/asm/syscall.h |    4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

@@ -101,10 +101,8 @@ static inline void syscall_get_arguments
 	/* O32 ABI syscall() - Either 64-bit with O32 or 32-bit */
 	if ((config_enabled(CONFIG_32BIT) ||
 	    test_tsk_thread_flag(task, TIF_32BIT_REGS)) &&
-	    (regs->regs[2] == __NR_syscall)) {
+	    (regs->regs[2] == __NR_syscall))
 		i++;
-		n++;
-	}
 
 	while (n--)
 		ret |= mips_get_syscall_arg(args++, task, regs, i++);

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/mips-fix-buffer-overflow-in-syscall_get_arguments.patch