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    Subject[PATCH AUTOSEL 5.4 004/350] drm/mipi-dbi: fix a loop in debugfs code
    Date
    From: Dan Carpenter <dan.carpenter@oracle.com>

    [ Upstream commit d72cf01f410aa09868d98b672f3f92328c96b32d ]

    This code will likely crash if we try to do a zero byte write. The code
    looks like this:

    /* strip trailing whitespace */
    for (i = count - 1; i > 0; i--)
    if (isspace(buf[i]))
    ...

    We're writing zero bytes so count = 0. You would think that "count - 1"
    would be negative one, but because "i" is unsigned it is a large
    positive numer instead. The "i > 0" condition is true and the "buf[i]"
    access will be out of bounds.

    The fix is to make "i" signed and now everything works as expected. The
    upper bound of "count" is capped in __kernel_write() at MAX_RW_COUNT so
    we don't have to worry about it being higher than INT_MAX.

    Fixes: 02dd95fe3169 ("drm/tinydrm: Add MIPI DBI support")
    Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
    [noralf: Adjust title]
    Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
    Link: https://patchwork.freedesktop.org/patch/msgid/20190821072456.GJ26957@mwanda
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    drivers/gpu/drm/drm_mipi_dbi.c | 3 +--
    1 file changed, 1 insertion(+), 2 deletions(-)

    diff --git a/drivers/gpu/drm/drm_mipi_dbi.c b/drivers/gpu/drm/drm_mipi_dbi.c
    index 1961f713aaab4..c4ee2709a6f32 100644
    --- a/drivers/gpu/drm/drm_mipi_dbi.c
    +++ b/drivers/gpu/drm/drm_mipi_dbi.c
    @@ -1187,8 +1187,7 @@ static ssize_t mipi_dbi_debugfs_command_write(struct file *file,
    struct mipi_dbi_dev *dbidev = m->private;
    u8 val, cmd = 0, parameters[64];
    char *buf, *pos, *token;
    - unsigned int i;
    - int ret, idx;
    + int i, ret, idx;

    if (!drm_dev_enter(&dbidev->drm, &idx))
    return -ENODEV;
    --
    2.20.1
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