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    Subject[PATCH 5.18 660/879] f2fs: fix dereference of stale list iterator after loop body
    Date
    From: Jakob Koschel <jakobkoschel@gmail.com>

    [ Upstream commit 2aaf51dd39afb6d01d13f1e6fe20b684733b37d5 ]

    The list iterator variable will be a bogus pointer if no break was hit.
    Dereferencing it (cur->page in this case) could load an out-of-bounds/undefined
    value making it unsafe to use that in the comparision to determine if the
    specific element was found.

    Since 'cur->page' *can* be out-ouf-bounds it cannot be guaranteed that
    by chance (or intention of an attacker) it matches the value of 'page'
    even though the correct element was not found.

    This is fixed by using a separate list iterator variable for the loop
    and only setting the original variable if a suitable element was found.
    Then determing if the element was found is simply checking if the
    variable is set.

    Fixes: 8c242db9b8c0 ("f2fs: fix stale ATOMIC_WRITTEN_PAGE private pointer")
    Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com>
    Reviewed-by: Chao Yu <chao@kernel.org>
    Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    fs/f2fs/segment.c | 9 ++++++---
    1 file changed, 6 insertions(+), 3 deletions(-)

    diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
    index bd9731cdec56..9dd9f88b75e9 100644
    --- a/fs/f2fs/segment.c
    +++ b/fs/f2fs/segment.c
    @@ -355,16 +355,19 @@ void f2fs_drop_inmem_page(struct inode *inode, struct page *page)
    struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
    struct list_head *head = &fi->inmem_pages;
    struct inmem_pages *cur = NULL;
    + struct inmem_pages *tmp;

    f2fs_bug_on(sbi, !page_private_atomic(page));

    mutex_lock(&fi->inmem_lock);
    - list_for_each_entry(cur, head, list) {
    - if (cur->page == page)
    + list_for_each_entry(tmp, head, list) {
    + if (tmp->page == page) {
    + cur = tmp;
    break;
    + }
    }

    - f2fs_bug_on(sbi, list_empty(head) || cur->page != page);
    + f2fs_bug_on(sbi, !cur);
    list_del(&cur->list);
    mutex_unlock(&fi->inmem_lock);

    --
    2.35.1


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