• Chen Jun's avatar
    mm/shmem.c: cast the type of unmap_start to u64 · aa71ecd8
    Chen Jun authored
    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.comSigned-off-by: default avatarChen Jun <chenjun102@huawei.com>
    Reviewed-by: default avatarAndrew 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: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    aa71ecd8
shmem.c 108 KB