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: [<ffff800000092150>] dump_backtrace+0x0/0x2c8 arch/arm64/kernel/traps.c:100 [<ffff800000092438>] show_stack+0x20/0x30 arch/arm64/kernel/traps.c:238 [<ffff800000f9b134>] __dump_stack lib/dump_stack.c:15 [inline] [<ffff800000f9b134>] ubsan_epilogue+0x18/0x70 lib/ubsan.c:164 [<ffff800000f9b468>] handle_overflow+0x158/0x1b0 lib/ubsan.c:195 [<ffff800000341280>] shmem_fallocate+0x6d0/0x820 mm/shmem.c:2104 [<ffff8000003ee008>] vfs_fallocate+0x238/0x428 fs/open.c:312 [<ffff8000003ef72c>] SYSC_fallocate fs/open.c:335 [inline] [<ffff8000003ef72c>] 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. Signed-off-by: Chen Jun <chenjun102@xxxxxxxxxx> --- mm/shmem.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mm/shmem.c b/mm/shmem.c index e9342c3..82cebbc 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2717,7 +2717,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset, } 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; -- 2.7.4
