[PATCH 1/5] mm: do not call vm_ops->access with FOLL_ANON

["David Hildenbrand (Arm)" <david@kernel.org>]

From: Chris Mason <mason@kernel.org>

Reading /proc/<pid>/cmdline or /proc/<pid>/environ can block in a device
driver, or read device memory, if the virtual addresses point at a device
mapping.

Ever since commit 7f7ccc2ccc2e ("proc: do not access cmdline nor environ
from file-backed areas") we use FOLL_ANON to only read from anonymous
memory. While GUP properly handles FOLL_ANON, __access_remote_vm()
will fall back on GUP errors to vma->vm_ops->access() without similarly
considering the flag.

Fix it by not calling vm_ops->access if FOLL_ANON was specified, to
truly limit it to anonymous memory only.

Fixes: 7f7ccc2ccc2e ("proc: do not access cmdline nor environ from file-backed areas")
Signed-off-by: Chris Mason <mason@kernel.org>
Cc: stable@vger.kernel.org
[ david: simplify description; slightly rework implementation ]
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/memory.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index 1a0ec09c56fd7..d4370c1b331c9 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -7398,6 +7398,9 @@ static int __access_remote_vm(struct mm_struct *mm, unsigned long addr,
 				continue;
 			}
 
+			if (gup_flags & FOLL_ANON)
+				break;
+
 			/*
 			 * Check if this is a VM_IO | VM_PFNMAP VMA, which
 			 * we can access using slightly different code.

-- 
2.43.0