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    Subject[PATCH 5.2 190/313] sched/psi: Correct overly pessimistic size calculation
    Date
    From: Miles Chen <miles.chen@mediatek.com>

    [ Upstream commit 4adcdcea717cb2d8436bef00dd689aa5bc76f11b ]

    When passing a equal or more then 32 bytes long string to psi_write(),
    psi_write() copies 31 bytes to its buf and overwrites buf[30]
    with '\0'. Which makes the input string 1 byte shorter than
    it should be.

    Fix it by copying sizeof(buf) bytes when nbytes >= sizeof(buf).

    This does not cause problems in normal use case like:
    "some 500000 10000000" or "full 500000 10000000" because they
    are less than 32 bytes in length.

    /* assuming nbytes == 35 */
    char buf[32];

    buf_size = min(nbytes, (sizeof(buf) - 1)); /* buf_size = 31 */
    if (copy_from_user(buf, user_buf, buf_size))
    return -EFAULT;

    buf[buf_size - 1] = '\0'; /* buf[30] = '\0' */

    Before:

    %cd /proc/pressure/
    %echo "123456789|123456789|123456789|1234" > memory
    [ 22.473497] nbytes=35,buf_size=31
    [ 22.473775] 123456789|123456789|123456789| (print 30 chars)
    %sh: write error: Invalid argument

    %echo "123456789|123456789|123456789|1" > memory
    [ 64.916162] nbytes=32,buf_size=31
    [ 64.916331] 123456789|123456789|123456789| (print 30 chars)
    %sh: write error: Invalid argument

    After:

    %cd /proc/pressure/
    %echo "123456789|123456789|123456789|1234" > memory
    [ 254.837863] nbytes=35,buf_size=32
    [ 254.838541] 123456789|123456789|123456789|1 (print 31 chars)
    %sh: write error: Invalid argument

    %echo "123456789|123456789|123456789|1" > memory
    [ 9965.714935] nbytes=32,buf_size=32
    [ 9965.715096] 123456789|123456789|123456789|1 (print 31 chars)
    %sh: write error: Invalid argument

    Also remove the superfluous parentheses.

    Signed-off-by: Miles Chen <miles.chen@mediatek.com>
    Cc: <linux-mediatek@lists.infradead.org>
    Cc: <wsd_upstream@mediatek.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Link: https://lkml.kernel.org/r/20190912103452.13281-1-miles.chen@mediatek.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/sched/psi.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
    index 6e52b67b420e7..517e3719027e6 100644
    --- a/kernel/sched/psi.c
    +++ b/kernel/sched/psi.c
    @@ -1198,7 +1198,7 @@ static ssize_t psi_write(struct file *file, const char __user *user_buf,
    if (static_branch_likely(&psi_disabled))
    return -EOPNOTSUPP;

    - buf_size = min(nbytes, (sizeof(buf) - 1));
    + buf_size = min(nbytes, sizeof(buf));
    if (copy_from_user(buf, user_buf, buf_size))
    return -EFAULT;

    --
    2.20.1


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