4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Michael Schmitz <schmitz...@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 <schmitz...@gmail.com>
Signed-off-by: Theodore Ts'o <ty...@mit.edu>
Signed-off-by: Sasha Levin <alexander.le...@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
 drivers/char/random.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1115,12 +1115,16 @@ static void add_interrupt_bench(cycles_t
 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)


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