Re: [PATCH] mm/mremap: Fix address wraparound in move_page_tables()

[Qi Zheng <zhengqi.arch@bytedance.com>]

On 2024/11/12 03:34, Jann Horn wrote:
> On 32-bit platforms, it is possible for the expression
> `len + old_addr < old_end` to be false-positive if `len + old_addr` wraps
> around. `old_addr` is the cursor in the old range up to which page table
> entries have been moved; so if the operation succeeded, `old_addr` is the
> *end* of the old region, and adding `len` to it can wrap.
> 
> The overflow causes mremap() to mistakenly believe that PTEs have been
> copied; the consequence is that mremap() bails out, but doesn't move the
> PTEs back before the new VMA is unmapped, causing anonymous pages in the
> region to be lost. So basically if userspace tries to mremap() a
> private-anon region and hits this bug, mremap() will return an error and
> the private-anon region's contents appear to have been zeroed.
> 
> The idea of this check is that `old_end - len` is the original start
> address, and writing the check that way also makes it easier to read; so
> fix the check by rearranging the comparison accordingly.
> 
> (An alternate fix would be to refactor this function by introducing an
> "orig_old_start" variable or such.)
> 
> Cc: stable@vger.kernel.org
> Fixes: af8ca1c14906 ("mm/mremap: optimize the start addresses in move_page_tables()")
> Signed-off-by: Jann Horn <jannh@google.com>

Acked-by: Qi Zheng <zhengqi.arch@bytedance.com>

Thanks!