From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michael Schmitz Subject: [RFC] fix race in drivers/char/random.c:get_reg() Date: Sun, 30 Apr 2017 19:49:21 +1200 Message-ID: <1493538561-5075-1-git-send-email-schmitzmic@gmail.com> Return-path: Received: from mail-pf0-f195.google.com ([209.85.192.195]:36715 "EHLO mail-pf0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S968649AbdD3Hta (ORCPT ); Sun, 30 Apr 2017 03:49:30 -0400 Sender: linux-m68k-owner@vger.kernel.org List-Id: linux-m68k@vger.kernel.org To: tytso@mit.edu, linux-kernel@vger.kernel.org Cc: linux-m68k@vger.kernel.org, Michael Schmitz get_reg() can be reentered on architectures with prioritized interrupts (m68k in this case), causing f->reg_index to be incremented after the range check. Out of bounds memory access past the pt_regs struct results. This will go mostly undetected unless access is beyond end of memory. Prevent the race by disabling interrupts in get_reg(). Tested on m68k (Atari Falcon, and ARAnyM emulator). Kudos to Geert Uytterhoeven for helping to trace this race. Signed-off-by: Michael Schmitz --- drivers/char/random.c | 6 +++++- 1 files changed, 5 insertions(+), 1 deletions(-) diff --git a/drivers/char/random.c b/drivers/char/random.c index 1ef2640..7d1799b 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -1115,12 +1115,16 @@ static void add_interrupt_bench(cycles_t start) static __u32 get_reg(struct fast_pool *f, struct pt_regs *regs) { __u32 *ptr = (__u32 *) regs; + unsigned long flags; if (regs == NULL) return 0; + local_irq_save(flags); if (f->reg_idx >= sizeof(struct pt_regs) / sizeof(__u32)) f->reg_idx = 0; - return *(ptr + f->reg_idx++); + ptr += f->reg_idx++; + local_irq_restore(flags); + return *ptr; } void add_interrupt_randomness(int irq, int irq_flags) -- 1.7.0.4