[PATCH AUTOSEL for 4.4 074/167] fix race in drivers/char/random.c:get_reg()

Sasha Levin
2018-03-19T16:09:33Z

From: Michael Schmitz <schmitzmic@gmail.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@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.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 1822472dffab..29b4218e9b63 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -886,12 +886,16 @@ 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