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SubjectRe: [PATCH v2] hugetlbfs: check for pgoff value overflow
On Thu,  8 Mar 2018 13:05:02 -0800 Mike Kravetz <mike.kravetz@oracle.com> wrote:

> A vma with vm_pgoff large enough to overflow a loff_t type when
> converted to a byte offset can be passed via the remap_file_pages
> system call. The hugetlbfs mmap routine uses the byte offset to
> calculate reservations and file size.
>
> A sequence such as:
> mmap(0x20a00000, 0x600000, 0, 0x66033, -1, 0);
> remap_file_pages(0x20a00000, 0x600000, 0, 0x20000000000000, 0);
> will result in the following when task exits/file closed,
> kernel BUG at mm/hugetlb.c:749!
> Call Trace:
> hugetlbfs_evict_inode+0x2f/0x40
> evict+0xcb/0x190
> __dentry_kill+0xcb/0x150
> __fput+0x164/0x1e0
> task_work_run+0x84/0xa0
> exit_to_usermode_loop+0x7d/0x80
> do_syscall_64+0x18b/0x190
> entry_SYSCALL_64_after_hwframe+0x3d/0xa2
>
> The overflowed pgoff value causes hugetlbfs to try to set up a
> mapping with a negative range (end < start) that leaves invalid
> state which causes the BUG.
>
> The previous overflow fix to this code was incomplete and did not
> take the remap_file_pages system call into account.
>
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -111,6 +111,7 @@ static void huge_pagevec_release(struct pagevec *pvec)
> static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
> {
> struct inode *inode = file_inode(file);
> + unsigned long ovfl_mask;
> loff_t len, vma_len;
> int ret;
> struct hstate *h = hstate_file(file);
> @@ -127,12 +128,16 @@ static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
> vma->vm_ops = &hugetlb_vm_ops;
>
> /*
> - * Offset passed to mmap (before page shift) could have been
> - * negative when represented as a (l)off_t.
> + * page based offset in vm_pgoff could be sufficiently large to
> + * overflow a (l)off_t when converted to byte offset.
> */
> - if (((loff_t)vma->vm_pgoff << PAGE_SHIFT) < 0)
> + ovfl_mask = (1UL << (PAGE_SHIFT + 1)) - 1;
> + ovfl_mask <<= ((sizeof(unsigned long) * BITS_PER_BYTE) -
> + (PAGE_SHIFT + 1));

That's a compile-time constant. The compiler will indeed generate an
immediate load, but I think it would be better to make the code look
more like we know that it's a constant, if you get what I mean.
Something like

/*
* If a pgoff_t is to be converted to a byte index, this is the max value it
* can have to avoid overflow in that conversion.
*/
#define PGOFF_T_MAX <long string of crap>

And I bet that this constant could be used elsewhere - surely it's a
very common thing to be checking for.


Also, the expression seems rather complicated. Why are we adding 1 to
PAGE_SHIFT? Isn't there a logical way of using PAGE_MASK?

The resulting constant is 0xfff8000000000000 on 64-bit. We could just
use along the lines of

1UL << (BITS_PER_LONG - PAGE_SHIFT - 1)

But why the -1? We should be able to handle a pgoff_t of
0xfff0000000000000 in this code?


Also, we later to

len = vma_len + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
/* check for overflow */
if (len < vma_len)
return -EINVAL;

which is ungainly: even if we passed the PGOFF_T_MAX test, there can
still be an overflow which we still must check for. Is that avoidable?
Probably not...

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