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    Subject[PATCH AUTOSEL 4.4 01/80] drm/bufs: Fix Spectre v1 vulnerability
    Date
    From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>

    [ Upstream commit a37805098900a6e73a55b3a43b7d3bcd987bb3f4 ]

    idx can be indirectly controlled by user-space, hence leading to a
    potential exploitation of the Spectre variant 1 vulnerability.

    This issue was detected with the help of Smatch:

    drivers/gpu/drm/drm_bufs.c:1420 drm_legacy_freebufs() warn: potential
    spectre issue 'dma->buflist' [r] (local cap)

    Fix this by sanitizing idx before using it to index dma->buflist

    Notice that given that speculation windows are large, the policy is
    to kill the speculation on the first load and not worry if it can be
    completed with a dependent load/store [1].

    [1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2

    Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
    Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
    Link: https://patchwork.freedesktop.org/patch/msgid/20181016095549.GA23586@embeddedor.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    drivers/gpu/drm/drm_bufs.c | 3 +++
    1 file changed, 3 insertions(+)

    diff --git a/drivers/gpu/drm/drm_bufs.c b/drivers/gpu/drm/drm_bufs.c
    index f1a204d253cc..ac22b8d86249 100644
    --- a/drivers/gpu/drm/drm_bufs.c
    +++ b/drivers/gpu/drm/drm_bufs.c
    @@ -36,6 +36,8 @@
    #include <drm/drmP.h>
    #include "drm_legacy.h"

    +#include <linux/nospec.h>
    +
    static struct drm_map_list *drm_find_matching_map(struct drm_device *dev,
    struct drm_local_map *map)
    {
    @@ -1332,6 +1334,7 @@ int drm_legacy_freebufs(struct drm_device *dev, void *data,
    idx, dma->buf_count - 1);
    return -EINVAL;
    }
    + idx = array_index_nospec(idx, dma->buf_count);
    buf = dma->buflist[idx];
    if (buf->file_priv != file_priv) {
    DRM_ERROR("Process %d freeing buffer not owned\n",
    --
    2.19.1
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