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    Subject[PATCH 5.3 177/180] mm/shmem.c: cast the type of unmap_start to u64
    Date
    From: Chen Jun <chenjun102@huawei.com>

    commit aa71ecd8d86500da6081a72da6b0b524007e0627 upstream.

    In 64bit system. sb->s_maxbytes of shmem filesystem is MAX_LFS_FILESIZE,
    which equal LLONG_MAX.

    If offset > LLONG_MAX - PAGE_SIZE, offset + len < LLONG_MAX in
    shmem_fallocate, which will pass the checking in vfs_fallocate.

    /* Check for wrap through zero too */
    if (((offset + len) > inode->i_sb->s_maxbytes) || ((offset + len) < 0))
    return -EFBIG;

    loff_t unmap_start = round_up(offset, PAGE_SIZE) in shmem_fallocate
    causes a overflow.

    Syzkaller reports a overflow problem in mm/shmem:

    UBSAN: Undefined behaviour in mm/shmem.c:2014:10
    signed integer overflow: '9223372036854775807 + 1' cannot be represented in type 'long long int'
    CPU: 0 PID:17076 Comm: syz-executor0 Not tainted 4.1.46+ #1
    Hardware name: linux, dummy-virt (DT)
    Call trace:
    dump_backtrace+0x0/0x2c8 arch/arm64/kernel/traps.c:100
    show_stack+0x20/0x30 arch/arm64/kernel/traps.c:238
    __dump_stack lib/dump_stack.c:15 [inline]
    ubsan_epilogue+0x18/0x70 lib/ubsan.c:164
    handle_overflow+0x158/0x1b0 lib/ubsan.c:195
    shmem_fallocate+0x6d0/0x820 mm/shmem.c:2104
    vfs_fallocate+0x238/0x428 fs/open.c:312
    SYSC_fallocate fs/open.c:335 [inline]
    SyS_fallocate+0x54/0xc8 fs/open.c:239

    The highest bit of unmap_start will be appended with sign bit 1
    (overflow) when calculate shmem_falloc.start:

    shmem_falloc.start = unmap_start >> PAGE_SHIFT.

    Fix it by casting the type of unmap_start to u64, when right shifted.

    This bug is found in LTS Linux 4.1. It also seems to exist in mainline.

    Link: http://lkml.kernel.org/r/1573867464-5107-1-git-send-email-chenjun102@huawei.com
    Signed-off-by: Chen Jun <chenjun102@huawei.com>
    Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Qian Cai <cai@lca.pw>
    Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    mm/shmem.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/mm/shmem.c
    +++ b/mm/shmem.c
    @@ -2730,7 +2730,7 @@ static long shmem_fallocate(struct file
    }

    shmem_falloc.waitq = &shmem_falloc_waitq;
    - shmem_falloc.start = unmap_start >> PAGE_SHIFT;
    + shmem_falloc.start = (u64)unmap_start >> PAGE_SHIFT;
    shmem_falloc.next = (unmap_end + 1) >> PAGE_SHIFT;
    spin_lock(&inode->i_lock);
    inode->i_private = &shmem_falloc;

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