On 11/20/2025 6:31 PM, Lance Yang wrote:
>
>
> On 2025/11/20 14:50, Shivank Garg wrote:
>> When MADV_COLLAPSE is called on file-backed mappings (e.g., executable
>> text sections), the pages may still be dirty from recent writes and
>> cause collapse to fail with -EINVAL. This is particularly problematic
>> for freshly copied executables on filesystems, where page cache folios
>> remain dirty until background writeback completes.
>>
>> The current code in collapse_file() triggers async writeback via
>> filemap_flush() and expects khugepaged to revisit the page later.
>> However, MADV_COLLAPSE is a synchronous operation where userspace
>> expects immediate results.
>>
>> Perform synchronous writeback in madvise_collapse() before attempting
>> collapse to avoid failing on first attempt.
>
> Thanks!
>
>>
>> Reported-by: Branden Moore <[email protected]>
>> Closes:
>> https://lore.kernel.org/all/[email protected]
>> Fixes: 34488399fa08 ("mm/madvise: add file and shmem support to
>> MADV_COLLAPSE")
>> Suggested-by: David Hildenbrand <[email protected]>
>> Signed-off-by: Shivank Garg <[email protected]>
>> ---
>> mm/khugepaged.c | 26 ++++++++++++++++++++++++++
>> 1 file changed, 26 insertions(+)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 97d1b2824386..066a332c76ad 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -22,6 +22,7 @@
>> #include <linux/dax.h>
>> #include <linux/ksm.h>
>> #include <linux/pgalloc.h>
>> +#include <linux/backing-dev.h>
>> #include <asm/tlb.h>
>> #include "internal.h"
>> @@ -2784,6 +2785,31 @@ int madvise_collapse(struct vm_area_struct *vma,
>> unsigned long start,
>> hstart = (start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
>> hend = end & HPAGE_PMD_MASK;
>> + /*
>> + * For file-backed VMAs, perform synchronous writeback to ensure
>> + * dirty folios are flushed before attempting collapse. This avoids
>> + * failing on the first attempt when freshly-written executable text
>> + * is still dirty in the page cache.
>> + */
>> + if (!vma_is_anonymous(vma) && vma->vm_file) {
>> + struct address_space *mapping = vma->vm_file->f_mapping;
>> +
>> + if (mapping_can_writeback(mapping)) {
>> + pgoff_t pgoff_start = linear_page_index(vma, hstart);
>> + pgoff_t pgoff_end = linear_page_index(vma, hend);
>> + loff_t lstart = (loff_t)pgoff_start << PAGE_SHIFT;
>> + loff_t lend = ((loff_t)pgoff_end << PAGE_SHIFT) - 1;
>
> It looks like we need to hold a reference to the file here before
> dropping the mmap lock :)
>
> file = get_file(vma->vm_file);
>
> Without it, the vma could be destroyed by a concurrent munmap() while
> we are waiting in filemap_write_and_wait_range(), leading to a UAF
> on mapping, IIUC ...
Excellent catch!
Thanks for saving me from this nasty bug. I'll be more careful on file ref
handling in next version.
Best Regards,
Shivank
>
>> +
>> + mmap_read_unlock(mm);
>> + mmap_locked = false;
>> +
>> + if (filemap_write_and_wait_range(mapping, lstart, lend)) {
>
> And drop the reference :)
>
> fput(file);
>
>
>> + last_fail = SCAN_FAIL;
>> + goto out_maybelock;
>> + }
>
> Same here :)
>
> fput(file);
>
>
>> + }
>> + }
>> +
>> for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
>> int result = SCAN_FAIL;
>>
>
> Cheers,
> Lance
