[PATCH v2] mm: thp: deny THP for files on anonymous inodes

[Deepanshu Kartikey <kartikey406@gmail.com>]

file_thp_enabled() incorrectly allows THP for files on anonymous inodes
(e.g. guest_memfd and secretmem). These files are created via
alloc_file_pseudo(), which does not call get_write_access() and leaves
inode->i_writecount at 0. Combined with S_ISREG(inode->i_mode) being
true, they appear as read-only regular files when
CONFIG_READ_ONLY_THP_FOR_FS is enabled, making them eligible for THP
collapse.

Anonymous inodes can never pass the inode_is_open_for_write() check
since their i_writecount is never incremented through the normal VFS
open path. The right thing to do is to exclude them from THP eligibility
altogether, since CONFIG_READ_ONLY_THP_FOR_FS was designed for real
filesystem files (e.g. shared libraries), not for pseudo-filesystem
inodes.

For guest_memfd, this allows khugepaged and MADV_COLLAPSE to create
large folios in the page cache via the collapse path, but the
guest_memfd fault handler does not support large folios. This triggers
WARN_ON_ONCE(folio_test_large(folio)) in kvm_gmem_fault_user_mapping().

For secretmem, collapse_file() tries to copy page contents through the
direct map, but secretmem pages are removed from the direct map. This
can result in a kernel crash:

    BUG: unable to handle page fault for address: ffff88810284d000
    RIP: 0010:memcpy_orig+0x16/0x130
    Call Trace:
     collapse_file
     hpage_collapse_scan_file
     madvise_collapse

Secretmem is not affected by the crash on upstream as the memory failure
recovery handles the failed copy gracefully, but it still triggers
confusing false memory failure reports:

    Memory failure: 0x106d96f: recovery action for clean unevictable
    LRU page: Recovered

Check IS_ANON_FILE(inode) in file_thp_enabled() to deny THP for all
anonymous inode files.

Link: https://syzkaller.appspot.com/bug?extid=33a04338019ac7e43a44
Link: https://lore.kernel.org/linux-mm/CAEvNRgHegcz3ro35ixkDw39ES8=U6rs6S7iP0gkR9enr7HoGtA@mail.gmail.com
Reported-by: syzbot+33a04338019ac7e43a44@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=33a04338019ac7e43a44
Fixes: 7fbb5e188248 ("mm: remove VM_EXEC requirement for THP eligibility")
Tested-by: syzbot+33a04338019ac7e43a44@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Deepanshu Kartikey <Kartikey406@gmail.com>
---
v2:
  - Use IS_ANON_FILE(inode) to deny THP for all anonymous inode files
    instead of checking for specific subsystems (David Hildenbrand)
  - Updated Fixes tag to 7fbb5e188248 which removed the VM_EXEC
    requirement that accidentally protected secretmem
  - Expanded commit message with implications for both guest_memfd
    and secretmem
---
 mm/huge_memory.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 40cf59301c21..d3beddd8cc30 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -94,6 +94,9 @@ static inline bool file_thp_enabled(struct vm_area_struct *vma)
 
 	inode = file_inode(vma->vm_file);
 
+	if (IS_ANON_FILE(inode))
+		return false;
+
 	return !inode_is_open_for_write(inode) && S_ISREG(inode->i_mode);
 }
 
-- 
2.43.0