[RFC PATCH] userfaultfd: allow registration of ranges below mmap_min_addr

["Denis M. Karpov" <komlomal@gmail.com>]

The current implementation of validate_range() in fs/userfaultfd.c
performs a hard check against mmap_min_addr without considering
capabilities, but the mmap() syscall uses security_mmap_addr()
which allows privileged processes (with CAP_SYS_RAWIO) to map below
mmap_min_addr. Furthermore, security_mmap_addr()->cap_mmap_addr() uses
dac_mmap_min_addr variable which can be changed with
/proc/sys/vm/mmap_min_addr.

Because userfaultfd uses a different check, UFFDIO_REGISTER may fail
with -EINVAL for valid memory areas that were successfully mapped
below mmap_min_addr even with appropriate capabilities.

This prevents apps like binary compilers from using UFFD for valid memory
regions mapped by application.

Replace the rigid mmap_min_addr check with security_mmap_addr() to align
userfaultfd with the standard kernel memory mapping security policy.

Signed-off-by: Denis M. Karpov <komlomal@gmail.com>

---
Initial RFC following the discussion on the [BUG] thread.
Link: https://lore.kernel.org/all/CADtiZd0tWysx5HMCUnOXfSHB7PXAuXg1Mh4eY_hUmH29S=sejg@mail.gmail.com/
---
 fs/userfaultfd.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index bdc84e521..dbfe5b2a0 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1238,15 +1238,13 @@ static __always_inline int validate_unaligned_range(
 		return -EINVAL;
 	if (!len)
 		return -EINVAL;
-	if (start < mmap_min_addr)
-		return -EINVAL;
 	if (start >= task_size)
 		return -EINVAL;
 	if (len > task_size - start)
 		return -EINVAL;
 	if (start + len <= start)
 		return -EINVAL;
-	return 0;
+	return security_mmap_addr(start);
 }
 
 static __always_inline int validate_range(struct mm_struct *mm,
-- 
2.47.3