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    Subject[PATCH AUTOSEL for 3.18 044/102] fix race in drivers/char/random.c:get_reg()
    Date
    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 d55156fc064d..73049abf4898 100644
    --- a/drivers/char/random.c
    +++ b/drivers/char/random.c
    @@ -863,12 +863,16 @@ 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
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