mm: disallow ioremap'ing ordinary RAM in generic_access_phys()

generic_access_phys() can end up getting called on CoW PFNMAP mappings,
resulting in it trying to ioremap anonymous folios, which especially
the x86 code doesn't enjoy, and which is in general just odd/wrong.

E.g., with a CoW mapping of a PCI BAR:

ioremap on RAM at 0x0000000045623000 - 0x0000000045623fff
  WARNING: arch/x86/mm/ioremap.c:216 at __ioremap_caller.isra.0+0x4c2/0x5f0
  Call Trace:
   generic_access_phys+0x130/0x4d0 mm/memory.c:7178
   kernfs_vma_access+0x1ce/0x280 fs/kernfs/file.c:437
   __access_remote_vm+0x58f/0x890 mm/memory.c:7256
   mem_rw+0x2a1/0x670 fs/proc/base.c:912

But more generically, we should disallow access to anything that looks
like ordinary RAM in the system: including allocating memory (e.g.,
alloc_pages) and mapping them in a VM_PFNMAP.

For pages with an online memmap, allow only pages marked as reserved
(PageReserved). This also includes memblock allocations, however,
distinguishing them from other MMIO is tricky, and nobody should really
be mapping them in a PFNMAP (except someone abusing /dev/mem, which we
do not care about).

So only allow access if there is no online memmap, or if the pages are
marked reserved. Reject other pages with -EINVAL.

Note that, just like other pfn_to_online_page() users, we won't care about
concurrent memory offlining; mapping memory that can concurrently get
offlined is highly dubious.

Link: https://lore.kernel.org/20261013-generic_access_phys-v1-1-b3963ef76e37@kernel.org
Fixes: 28b2ee20c7cb ("access_process_vm device memory infrastructure")
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: <syzbot+49b1021becba70c1f3f6@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=49b1021becba70c1f3f6
Reported-by: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>
Closes: https://lore.kernel.org/r/20261001152524.171115-1-ngocthang2710.1999@gmail.com/
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Tested-by: Lance Yang <lance.yang@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Rik van Riel <riel@redhat.com>
Cc: <stable@vger.kernel.org>

0 files changed, 0 insertions, 0 deletions