[PATCH v5 20/25] fs: Handle intra-page faults in copy_mount_options()

Catalin Marinas
2020-06-24T17:53:34Z

The copy_mount_options() function takes a user pointer argument but no
size. 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 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 returning
-EFAULT only if the first copy_from_user() has not copied any bytes.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Reviewed-by: Kevin Brodsky <kevin.brodsky@arm.com>
---

Notes:
v4:
- Rewrite to avoid arch_has_exact_copy_from_user()

New in v3.

fs/namespace.c | 24 +++++++++++++++++++++---
1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index f30ed401cc6d..5b6a9c459674 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 size, left;

if (!data)
return NULL;
@@ -3085,12 +3085,30 @@ void *copy_mount_options(const void __user * data)

size = PAGE_SIZE - offset_in_page(data);

- if (copy_from_user(copy, data, size)) {
+ /*
+ * Attempt to copy to the end of the first user page. On success,
+ * left == 0, copy the rest from the second user page (if it is
+ * accessible). copy_from_user() will zero the part of the kernel
+ * buffer not copied into.
+ *
+ * On architectures with intra-page faults (arm64 with MTE), the read
+ * from the first page may fail after copying part of the user data
+ * (left > 0 && left < size). Do not attempt the second copy in this
+ * case as the end of the valid user buffer has already been reached.
+ * Ensure, however, that the second part of the kernel buffer is
+ * zeroed.
+ */
+ left = copy_from_user(copy, data, size);
+ if (left == size) {
kfree(copy);
return ERR_PTR(-EFAULT);
}
if (size != PAGE_SIZE) {
- if (copy_from_user(copy + size, data + size, PAGE_SIZE - size))
+ if (left == 0)
+ /* return not relevant, just silence the compiler */
+ left = copy_from_user(copy + size, data + size,
+ PAGE_SIZE - size);
+ else
memset(copy + size, 0, PAGE_SIZE - size);
}
return copy;