[PATCH 2/3] mm/mlock: reject ranges that cannot be represented

["Jose A. Perez de Azpillaga" <azpijr@gmail.com>]

A length within a page of ULONG_MAX wraps when mlock() and munlock()
round the range to pages. At an aligned address the length becomes 0 and
the call is a silent success; at an unaligned one it becomes a single
page, which is then locked or unlocked. An overflowing start + len is
only caught in apply_vma_lock_flags(), after the RLIMIT_MEMLOCK check, so
mlock() returns -ENOMEM without CAP_IPC_LOCK and -EINVAL with it.

Return -EINVAL, as man 2 mlock documents, when start + len overflows or
the end overflows when rounded up to a page. Doing this before
can_do_mlock() also avoids making the result depend on
RLIMIT_MEMLOCK/CAP_IPC_LOCK. Requests that get a silent success today
now fail.

Reported-by: Park Tae-sun <ts930@dgu.ac.kr>
Closes: https://lore.kernel.org/all/179066531783.50175.13377521828379196177@dgu.ac.kr/
Signed-off-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>
---
 mm/mlock.c | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/mm/mlock.c b/mm/mlock.c
index cc28e5d7413a..a9e2b90c058f 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -608,6 +608,7 @@ static __must_check int do_mlock(unsigned long start, size_t len,
 {
 	unsigned long locked;
 	unsigned long lock_limit;
+	unsigned long end;
 	int error = -ENOMEM;

 	start = untagged_addr(start);
@@ -615,12 +616,19 @@ static __must_check int do_mlock(unsigned long start, size_t len,
 	if (!len)
 		return 0;

+	/* Reject a wrapping range before can_do_mlock() and the rlimit check. */
+	end = start + len;
+	if (end < start)
+		return -EINVAL;
+	end = PAGE_ALIGN(end);
+	if (!end)
+		return -EINVAL;
+	start &= PAGE_MASK;
+	len = end - start;
+
 	if (!can_do_mlock())
 		return -EPERM;

-	len = PAGE_ALIGN(len + (offset_in_page(start)));
-	start &= PAGE_MASK;
-
 	lock_limit = rlimit(RLIMIT_MEMLOCK);
 	lock_limit >>= PAGE_SHIFT;
 	locked = len >> PAGE_SHIFT;
@@ -676,6 +684,7 @@ SYSCALL_DEFINE3(mlock2, unsigned long, start, size_t, len, int, flags)

 SYSCALL_DEFINE2(munlock, unsigned long, start, size_t, len)
 {
+	unsigned long end;
 	vma_flags_t flags = EMPTY_VMA_FLAGS;
 	int ret;

@@ -684,8 +693,14 @@ SYSCALL_DEFINE2(munlock, unsigned long, start, size_t, len)
 	if (!len)
 		return 0;

-	len = PAGE_ALIGN(len + (offset_in_page(start)));
+	end = start + len;
+	if (end < start)
+		return -EINVAL;
+	end = PAGE_ALIGN(end);
+	if (!end)
+		return -EINVAL;
 	start &= PAGE_MASK;
+	len = end - start;

 	if (mmap_write_lock_killable(current->mm))
 		return -EINTR;
--
2.55.0