mm/mlock: use a long for the page count in mlock_fixup()

mlock_fixup() adjusts mm->locked_vm, an unsigned long, by the number of
pages whose VM_LOCKED state it changes, but computes that number in an
int:

	int nr_pages;
	...
	nr_pages = (end - start) >> PAGE_SHIFT;
	...
	mm->locked_vm += nr_pages;

When a single VMA range of 2^31 pages (8 TiB with 4 KiB pages) or more
is locked or unlocked, the count is truncated to 32 bits and then
sign-extended, so locked_vm is changed by the wrong amount, while
munmap(), mremap() and the other paths that maintain locked_vm use the
full unsigned long page count.

Such a range is reachable on 64-bit architectures with a 47-bit or
larger user address space (x86_64, arm64 with 48-bit VA, riscv64 Sv48)
by a task with CAP_IPC_LOCK or a raised RLIMIT_MEMLOCK, which do_mlock()
checks against the full untruncated length.  It costs nothing with
MAP_NORESERVE and MLOCK_ONFAULT/MCL_ONFAULT, since then no page is
populated.  On x86_64, after

	p = mmap(NULL, 8UL << 40, PROT_READ | PROT_WRITE,
		 MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE, -1, 0);
	mlock2(p, 8UL << 40, MLOCK_ONFAULT);

VmLck in /proc/self/status reads 18446744065119617024 kB instead of
8589934592 kB; munmap() of the region then subtracts the true size and
leaves VmLck at 18446744056529682432 kB instead of 0.  With 16 TiB,
locked_vm is not changed at all, VmLck stays 0 kB, and after the task
drops CAP_IPC_LOCK or lowers RLIMIT_MEMLOCK, mlock(), mmap(MAP_LOCKED)
and the other mlock_future_ok() and account_locked_vm() users enforce
RLIMIT_MEMLOCK against that count, so they let the task lock memory
beyond its limit (or, in the 8 TiB case, refuse every further lock).
The error persists for the life of the mm; unlocking the range in pieces
skews it further.  The wrong count is per-mm accounting only: it does
not change which pages are locked.

The int has been there since before git history (the variable was named
"pages" until it was renamed in 2.6.28); every other updater of
locked_vm uses an unsigned long or long count.  Make nr_pages a long,
which holds the page count of any user range and its negation, so the
existing sign handling is unchanged.

Found by formal verification of mm/mlock.c against the kernel binary.

Link: https://lore.kernel.org/20261014131358.780240-1-nickolai@csail.mit.edu
Fixes: 1da177e4c3 ("Linux-2.6.12-rc2")
Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Assisted-by: LLM
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: <stable@vger.kernel.org>

mm/mlock.c

@@ -457,7 +457,7 @@ static int mlock_fixup(struct vma_iterator *vmi, struct vm_area_struct *vma,
 {
 	const vma_flags_t old_vma_flags = vma->flags;
 	struct mm_struct *mm = vma->vm_mm;
-	int nr_pages;
+	long nr_pages;
 	int ret = 0;
 
 	if (vma_flags_same_pair(&old_vma_flags, new_vma_flags) ||