Commit d563d678 authored Jul 01, 2020 by Catalin Marinas

fs: Handle intra-page faults in copy_mount_options()

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@arm.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>

parent 2200aa71

fs/namespace.c

+18 −7

Original line number	Diff line number	Diff line
		@@ -3075,7 +3075,7 @@ 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;
		@@ -3084,16 +3084,27 @@ 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;
		}

Admin message