On 9/11/19 8:18 PM, Christian Barcenas wrote:
A process can lock memory addresses into physical RAM explicitly
(via mlock, mlockall, shmctl, etc.) or implicitly (via VFIO,
perf ring-buffers, bpf maps, etc.), subject to RLIMIT_MEMLOCK limits.
CAP_IPC_LOCK allows a process to exceed these limits, and throughout
the kernel this capability is checked before allowing/denying an attempt
to lock memory regions into RAM.
Because bpf locks its programs and maps into RAM, it should respect
CAP_IPC_LOCK. Previously, bpf would return EPERM when RLIMIT_MEMLOCK was
exceeded by a privileged process, which is contrary to documented
RLIMIT_MEMLOCK+CAP_IPC_LOCK behavior.
Do you have a link/pointer where this is /clearly/ documented?
I admit that after submitting this patch, I did re-think the description
and thought maybe I should have described the CAP_IPC_LOCK behavior as
"expected" rather than "documented". :)
... but my best guess is you are referring to `man 2 mlock`:
Limits and permissions
In Linux 2.6.8 and earlier, a process must be privileged
(CAP_IPC_LOCK)
in order to lock memory and the RLIMIT_MEMLOCK soft resource
limit defines
a limit on how much memory the process may lock.
Since Linux 2.6.9, no limits are placed on the amount of
memory that a
privileged process can lock and the RLIMIT_MEMLOCK soft resource
limit
instead defines a limit on how much memory an unprivileged
process may lock.
Yes; this is what I was referring to by "documented
RLIMIT_MEMLOCK+CAP_IPC_LOCK behavior."
Unfortunately - AFAICT - this is the most explicit documentation about
CAP_IPC_LOCK's permission set, but it is incomplete.
I believe it can be understood from other references to RLIMIT and
CAP_IPC_LOCK throughout the kernel that "locking memory" refers not only
to mlock/shmctl syscalls, but also to other code sites where /physical/
memory addresses are allocated for userspace.
After identifying RLIMIT_MEMLOCK checks with
git grep -C3 '[^(get|set)]rlimit(RLIMIT_MEMLOCK'
we find that RLIMIT_MEMLOCK is bypassed - if CAP_IPC_LOCK is held - in
many locations that have nothing to do with the mlock or shm family of
syscalls. From what I can tell, every time RLIMIT_MEMLOCK is referenced
there is a neighboring check to CAP_IPC_LOCK that bypasses the rlimit,
or in some cases memory accounting entirely!
bpf() is currently the only exception to the above, ie. as far as I can
tell it is the only code that enforces RLIMIT_MEMLOCK but does not honor
CAP_IPC_LOCK.
Selected examples follow:
In net/core/skbuff.c:
if (capable(CAP_IPC_LOCK) || !size)
return 0;
num_pg = (size >> PAGE_SHIFT) + 2; /* worst case */
max_pg = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
user = mmp->user ? : current_user();
do {
old_pg = atomic_long_read(&user->locked_vm);
new_pg = old_pg + num_pg;
if (new_pg > max_pg)
return -ENOBUFS;
} while (atomic_long_cmpxchg(&user->locked_vm, old_pg, new_pg) !=
old_pg);
In net/xdp/xdp_umem.c:
if (capable(CAP_IPC_LOCK))
return 0;
lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
umem->user = get_uid(current_user());
do {
old_npgs = atomic_long_read(&umem->user->locked_vm);
new_npgs = old_npgs + umem->npgs;
if (new_npgs > lock_limit) {
free_uid(umem->user);
umem->user = NULL;
return -ENOBUFS;
}
} while (atomic_long_cmpxchg(&umem->user->locked_vm, old_npgs,
new_npgs) != old_npgs);
return 0;
In arch/x86/kvm/svm.c:
lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
if (locked > lock_limit && !capable(CAP_IPC_LOCK)) {
pr_err("SEV: %lu locked pages exceed the lock limit of
%lu.\n", locked, lock_limit);
return NULL;
}
In drivers/infiniband/core/umem.c (and other sites in Infiniband code):
lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
new_pinned = atomic64_add_return(npages, &mm->pinned_vm);
if (new_pinned > lock_limit && !capable(CAP_IPC_LOCK)) {
atomic64_sub(npages, &mm->pinned_vm);
ret = -ENOMEM;
goto out;
}
In drivers/vfio/vfio_iommu_type1.c, albeit in an indirect way:
struct vfio_dma {
bool lock_cap; /* capable(CAP_IPC_LOCK) */
};
// ...
for (vaddr += PAGE_SIZE, iova += PAGE_SIZE; pinned < npage;
pinned++, vaddr += PAGE_SIZE, iova += PAGE_SIZE) {
// ...
if (!rsvd && !vfio_find_vpfn(dma, iova)) {
if (!dma->lock_cap &&
current->mm->locked_vm + lock_acct + 1 > limit) {
put_pfn(pfn, dma->prot);
pr_warn("%s: RLIMIT_MEMLOCK (%ld) exceeded\n",
__func__, limit << PAGE_SHIFT);
ret = -ENOMEM;
goto unpin_out;
}
lock_acct++;
}
}
Best,
Christian
