[PATCH v7 24/29] fs: Handle intra-page faults in copy_mount_options()

[Catalin Marinas <catalin.marinas@xxxxxxx>]

The copy_mount_options() function takes a user pointer argument but no
size and it tries to read up to a PAGE_SIZE. However, copy_from_user()
is not guaranteed to return all the accessible bytes if, for example,
the access crosses a page boundary and gets a fault on the second page.
To work around this, the current copy_mount_options() implementation
performs two copy_from_user() passes, first to the end of the current
page and the second to what's left in the subsequent page.

On arm64 with MTE enabled, access to a user page may trigger a fault
after part of the buffer in a page has been copied (when the user
pointer tag, bits 56-59, no longer matches the allocation tag stored in
memory). Allow copy_mount_options() to handle such intra-page faults by
resorting to byte at a time copy in case of copy_from_user() failure.

Note that copy_from_user() handles the zeroing of the kernel buffer in
case of error.

Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
---

Al, I thought I'd not spam you with the whole arm64 MTE series, so only
cc'ing you on a patch in the middle. Would you mind ack'ing/nak'ing this
patch? I intend to push it upstream with the rest of the series for 5.9
(well, assuming nothing falls apart). It's not as elegant as the
previous approach but with MTE we can get faults in the middle of a
page, so we'd have to fall back to byte by byte copying.

Thanks.

Notes:
    v6:
    - Simplified logic to fall-back to byte-by-byte if the copy_from_user()
      fails.
    
    v4:
    - Rewrite to avoid arch_has_exact_copy_from_user()
    
    New in v3.

 fs/namespace.c | 25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index f30ed401cc6d..fc45020c244e 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3074,7 +3074,7 @@ static void shrink_submounts(struct mount *mnt)
 void *copy_mount_options(const void __user * data)
 {
 	char *copy;
-	unsigned size;
+	unsigned left, offset;
 
 	if (!data)
 		return NULL;
@@ -3083,16 +3083,27 @@ void *copy_mount_options(const void __user * data)
 	if (!copy)
 		return ERR_PTR(-ENOMEM);
 
-	size = PAGE_SIZE - offset_in_page(data);
+	left = copy_from_user(copy, data, PAGE_SIZE);
 
-	if (copy_from_user(copy, data, size)) {
+	/*
+	 * Not all architectures have an exact copy_from_user(). Resort to
+	 * byte at a time.
+	 */
+	offset = PAGE_SIZE - left;
+	while (left) {
+		char c;
+		if (get_user(c, (const char __user *)data + offset))
+			break;
+		copy[offset] = c;
+		left--;
+		offset++;
+	}
+
+	if (left == PAGE_SIZE) {
 		kfree(copy);
 		return ERR_PTR(-EFAULT);
 	}
-	if (size != PAGE_SIZE) {
-		if (copy_from_user(copy + size, data + size, PAGE_SIZE - size))
-			memset(copy + size, 0, PAGE_SIZE - size);
-	}
+
 	return copy;
 }