mm: fs: initialize fsdata passed to write_begin/write_end interface

Functions implementing the a_ops->write_end() interface accept the `void
*fsdata` parameter that is supposed to be initialized by the corresponding
a_ops->write_begin() (which accepts `void **fsdata`).

However not all a_ops->write_begin() implementations initialize `fsdata`
unconditionally, so it may get passed uninitialized to a_ops->write_end(),
resulting in undefined behavior.

Fix this by initializing fsdata with NULL before the call to
write_begin(), rather than doing so in all possible a_ops implementations.

This patch covers only the following cases found by running x86 KMSAN
under syzkaller:

 - generic_perform_write()
 - cont_expand_zero() and generic_cont_expand_simple()
 - page_symlink()

Other cases of passing uninitialized fsdata may persist in the codebase.

Link: https://lkml.kernel.org/r/20220915150417.722975-43-glider@google.com
Signed-off-by: Alexander Potapenko <glider@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Eric Biggers <ebiggers@google.com>
Cc: Eric Biggers <ebiggers@kernel.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Marco Elver <elver@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Vegard Nossum <vegard.nossum@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/buffer.c b/fs/buffer.c
index b4c9fff3ab6c941a1f4de3393bd9a980f560fb3f..b55252078e7bf67ff50f7aa1017ecbef2b069581 100644 (file)
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2341,7 +2341,7 @@ int generic_cont_expand_simple(struct inode *inode, loff_t size)
        struct address_space *mapping = inode->i_mapping;
        const struct address_space_operations *aops = mapping->a_ops;
        struct page *page;
-       void *fsdata;
+       void *fsdata = NULL;
        int err;
 
        err = inode_newsize_ok(inode, size);
@@ -2367,7 +2367,7 @@ static int cont_expand_zero(struct file *file, struct address_space *mapping,
        const struct address_space_operations *aops = mapping->a_ops;
        unsigned int blocksize = i_blocksize(inode);
        struct page *page;
-       void *fsdata;
+       void *fsdata = NULL;
        pgoff_t index, curidx;
        loff_t curpos;
        unsigned zerofrom, offset, len;

diff --git a/fs/namei.c b/fs/namei.c
index 53b4bc094db23a7be059f4b27aa1e5750db733ed..076ae96ca0b14cc4aa63b7a8a2b945d1cb4fff2e 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -5088,7 +5088,7 @@ int page_symlink(struct inode *inode, const char *symname, int len)
        const struct address_space_operations *aops = mapping->a_ops;
        bool nofs = !mapping_gfp_constraint(mapping, __GFP_FS);
        struct page *page;
-       void *fsdata;
+       void *fsdata = NULL;
        int err;
        unsigned int flags;
 

diff --git a/mm/filemap.c b/mm/filemap.c
index f27c93a581ab4a4fc9ba3f0f3d2a084791c976a9..ec17bd1a3bb7735de57c5af2400686d0521d5547 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3720,7 +3720,7 @@ ssize_t generic_perform_write(struct kiocb *iocb, struct iov_iter *i)
                unsigned long offset;   /* Offset into pagecache page */
                unsigned long bytes;    /* Bytes to write to page */
                size_t copied;          /* Bytes copied from user */
-               void *fsdata;
+               void *fsdata = NULL;
 
                offset = (pos & (PAGE_SIZE - 1));
                bytes = min_t(unsigned long, PAGE_SIZE - offset,