linux-kernel - [PATCH AUTOSEL for 4.9 107/281] fix race in drivers/char/random.c:get

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Date:   Mon, 19 Mar 2018 15:59:59 +0000
From:   Sasha Levin <Alexander.Levin@...rosoft.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
CC:     Michael Schmitz <schmitzmic@...il.com>,
        Theodore Ts'o <tytso@....edu>,
        Sasha Levin <Alexander.Levin@...rosoft.com>
Subject: [PATCH AUTOSEL for 4.9 107/281] fix race in
 drivers/char/random.c:get_reg()

From: Michael Schmitz <schmitzmic@...il.com>

[ Upstream commit 9dfa7bba35ac08a63565d58c454dccb7e1bb0a08 ]

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 <schmitzmic@...il.com>
Signed-off-by: Theodore Ts'o <tytso@....edu>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
---
 drivers/char/random.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 08d1dd58c0d2..23f3fb45ff07 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)
-- 
2.14.1