Re: [PATCH v1] mm/mlock: use a long for the page count in mlock_fixup()

[Pedro Falcato]

On Wed, Oct 14, 2026 at 09:13:58AM -0400, Nickolai Zeldovich wrote:
> 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.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Assisted-by: LLM
> Signed-off-by: Nickolai Zeldovich <nickolai@xxxxxxxxxxxxx>

Reviewed-by: Pedro Falcato <pfalcato@xxxxxxx>

LGTM, thanks!

-- 
Pedro