[RFC][PATCH] afs: Work around strnlen() oops with CONFIG_FORTIFIED_SOURCE=y

[David Howells]

Linus Torvalds <torvalds at linux-foundation.org> wrote:

> I htink the right fix would be to try to create a type that actually
> describes that.

How about the attached, then?  It's basically what you suggested.

I realised whilst doing this that I'm not quite getting the extended-name
handling correct, but I'll deal with that in a separate patch.

David
---
commit 26982a89cad77c0efc1c0c79bee0e3d75e9281d4
Author: David Howells <dhowells at redhat.com>
Date:   Mon Dec 21 22:37:58 2020 +0000

    afs: Work around strnlen() oops with CONFIG_FORTIFIED_SOURCE=y
    
    AFS has a structured layout in its directory contents (AFS dirs are
    downloaded as files and parsed locally by the client for lookup/readdir).
    The slots in the directory are defined by union afs_xdr_dirent.  This,
    however, only directly allows a name of a length that will fit into that
    union.  To support a longer name, the next 1-8 contiguous entries are
    annexed to the first one and the name flows across these.
    
    afs_dir_iterate_block() uses strnlen(), limited to the space to the end of
    the page, to find out how long the name is.  This worked fine until
    6a39e62abbaf.  With that commit, the compiler determines the size of the
    array and asserts that the string fits inside that array.  This is a
    problem for AFS because we *expect* it to overflow one or more arrays.
    
    A similar problem also occurs in afs_dir_scan_block() when a directory file
    is being locally edited to avoid the need to redownload it.  There strlen()
    was being used safely because each page has the last byte set to 0 when the
    file is downloaded and validated (in afs_dir_check_page()).
    
    Fix this by changing the afs_xdr_dirent union name field to an
    indeterminate-length array and dropping the overflow field.
    
    (Note that whilst looking at this, I realised that the calculation of the
    number of slots a dirent used is non-standard and not quite right, but I'll
    address that in a separate patch.)
    
    The issue can be triggered by something like:
    
            touch /afs/example.com/thisisaveryveryverylongname
    
    and it generates a report that looks like:
    
            detected buffer overflow in strnlen
            ------------[ cut here ]------------
            kernel BUG at lib/string.c:1149!
            ...
            RIP: 0010:fortify_panic+0xf/0x11
            ...
            Call Trace:
             afs_dir_iterate_block+0x12b/0x35b
             afs_dir_iterate+0x14e/0x1ce
             afs_do_lookup+0x131/0x417
             afs_lookup+0x24f/0x344
             lookup_open.isra.0+0x1bb/0x27d
             open_last_lookups+0x166/0x237
             path_openat+0xe0/0x159
             do_filp_open+0x48/0xa4
             ? kmem_cache_alloc+0xf5/0x16e
             ? __clear_close_on_exec+0x13/0x22
             ? _raw_spin_unlock+0xa/0xb
             do_sys_openat2+0x72/0xde
             do_sys_open+0x3b/0x58
             do_syscall_64+0x2d/0x3a
             entry_SYSCALL_64_after_hwframe+0x44/0xa9
    
    Fixes: 6a39e62abbaf ("lib: string.h: detect intra-object overflow in fortified string functions")
    Reported-by: Marc Dionne <marc.dionne at auristor.com>
    Signed-off-by: David Howells <dhowells at redhat.com>
    Tested-by: Marc Dionne <marc.dionne at auristor.com>
    cc: Daniel Axtens <dja at axtens.net>

diff --git a/fs/afs/xdr_fs.h b/fs/afs/xdr_fs.h
index 94f1f398eefa..c926430fd08a 100644
--- a/fs/afs/xdr_fs.h
+++ b/fs/afs/xdr_fs.h
@@ -54,10 +54,15 @@ union afs_xdr_dirent {
 		__be16		hash_next;
 		__be32		vnode;
 		__be32		unique;
-		u8		name[16];
-		u8		overflow[4];	/* if any char of the name (inc
-						 * NUL) reaches here, consume
-						 * the next dirent too */
+		u8		name[];
+		/* When determining the number of dirent slots needed to
+		 * represent a directory entry, name should be assumed to be 16
+		 * bytes, due to a now-standardised (mis)calculation, but it is
+		 * in fact 20 bytes in size.
+		 *
+		 * For names longer than (16 or) 20 bytes, extra slots should
+		 * be annexed to this one using the extended_name format.
+		 */
 	} u;
 	u8			extended_name[32];
 } __packed;