[PATCH v4 10/10] cachefiles: Fix potential UAF/KASAN warning

[David Howells <dhowells@redhat.com>]

Currently, trace_cachefiles_coherency() is being passed a pointer to a
__be64 lain over the coherency data in struct cachefiles_xattr so that it
can display the first 8 bytes.  However, the data is of variable length and
could even be 0 bytes.  This could lead to a UAF or KASAN warning.

Fix this by making sure the buffer has room for at least 8 bytes and that
those 8 bytes are pre-cleared.

Further, those bytes are not 8-byte aligned, so fix the tracepoint to
extract the data as four 2-byte words (they are 2-byte aligned) and
reassemble the __be64.  The compiler will convert this into a single 8-byte
load where the CPU supports it.

Fixes: 229105e5cfd9 ("cachefiles: Add auxiliary data trace")
Link: https://sashiko.dev/#/patchset/20260810144746.574036-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Paulo Alcantara <pc@manguebit.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
---
 fs/cachefiles/xattr.c             | 16 ++++++++--------
 include/trace/events/cachefiles.h | 19 +++++++++++++++++--
 2 files changed, 25 insertions(+), 10 deletions(-)

diff --git a/fs/cachefiles/xattr.c b/fs/cachefiles/xattr.c
index f8ae78b3f7b6..c70bf67e52b0 100644
--- a/fs/cachefiles/xattr.c
+++ b/fs/cachefiles/xattr.c
@@ -13,6 +13,7 @@
 #include <linux/quotaops.h>
 #include <linux/xattr.h>
 #include <linux/slab.h>
+#include <linux/unaligned.h>
 #include "internal.h"
 
 #define CACHEFILES_COOKIE_TYPE_DATA 1
@@ -50,7 +51,7 @@ int cachefiles_set_object_xattr(struct cachefiles_object *object)
 
 	_enter("%x,#%d", object->debug_id, len);
 
-	buf = kmalloc(sizeof(struct cachefiles_xattr) + len, GFP_KERNEL);
+	buf = kmalloc(sizeof(struct cachefiles_xattr) + max(len, sizeof(__be64)), GFP_KERNEL);
 	if (!buf)
 		return -ENOMEM;
 
@@ -60,6 +61,7 @@ int cachefiles_set_object_xattr(struct cachefiles_object *object)
 	buf->content		= object->content_info;
 	if (test_bit(FSCACHE_COOKIE_LOCAL_WRITE, &object->cookie->flags))
 		buf->content	= CACHEFILES_CONTENT_DIRTY;
+	put_unaligned_be64(0, (__be64 *)buf->data);
 	if (len > 0)
 		memcpy(buf->data, fscache_get_aux(object->cookie), len);
 
@@ -77,8 +79,7 @@ int cachefiles_set_object_xattr(struct cachefiles_object *object)
 		trace_cachefiles_vfs_error(object, file_inode(file), ret,
 					   cachefiles_trace_setxattr_error);
 		trace_cachefiles_coherency(object, file_inode(file)->i_ino,
-					   be64_to_cpup((__be64 *)buf->data),
-					   buf->content,
+					   buf->data, buf->content,
 					   cachefiles_coherency_set_fail);
 		if (ret != -ENOMEM)
 			cachefiles_io_error_obj(
@@ -86,8 +87,7 @@ int cachefiles_set_object_xattr(struct cachefiles_object *object)
 				"Failed to set xattr with error %d", ret);
 	} else {
 		trace_cachefiles_coherency(object, file_inode(file)->i_ino,
-					   be64_to_cpup((__be64 *)buf->data),
-					   buf->content,
+					   buf->data, buf->content,
 					   cachefiles_coherency_set_ok);
 	}
 
@@ -110,9 +110,10 @@ int cachefiles_check_auxdata(struct cachefiles_object *object, struct file *file
 	int ret = -ESTALE;
 
 	tlen = sizeof(struct cachefiles_xattr) + len;
-	buf = kmalloc(tlen, GFP_KERNEL);
+	buf = kmalloc(sizeof(struct cachefiles_xattr) + max(len, sizeof(__be64)), GFP_KERNEL);
 	if (!buf)
 		return -ENOMEM;
+	put_unaligned_be64(0, (__be64 *)buf->data);
 
 	xlen = cachefiles_inject_read_error();
 	if (xlen == 0)
@@ -148,8 +149,7 @@ int cachefiles_check_auxdata(struct cachefiles_object *object, struct file *file
 
 out:
 	trace_cachefiles_coherency(object, file_inode(file)->i_ino,
-				   be64_to_cpup((__be64 *)buf->data),
-				   buf->content, why);
+				   buf->data, buf->content, why);
 	kfree(buf);
 	return ret;
 }
diff --git a/include/trace/events/cachefiles.h b/include/trace/events/cachefiles.h
index 9259bc71049e..e3101410e8b2 100644
--- a/include/trace/events/cachefiles.h
+++ b/include/trace/events/cachefiles.h
@@ -372,7 +372,7 @@ TRACE_EVENT(cachefiles_rename,
 TRACE_EVENT(cachefiles_coherency,
 	    TP_PROTO(struct cachefiles_object *obj,
 		     ino_t ino,
-		     u64 disk_aux,
+		     const void *disk_aux,
 		     enum cachefiles_content content,
 		     enum cachefiles_coherency_trace why),
 
@@ -389,12 +389,27 @@ TRACE_EVENT(cachefiles_coherency,
 			     ),
 
 	    TP_fast_assign(
+		    union {
+			    __be16 s[4];
+			    __be64 ll;
+		    } x;
+
 		    __entry->obj	= obj->debug_id;
 		    __entry->why	= why;
 		    __entry->content	= content;
 		    __entry->ino	= ino;
 		    __entry->aux	= be64_to_cpup((__be64 *)obj->cookie->inline_aux);
-		    __entry->disk_aux	= disk_aux;
+
+		    /* cachefiles_xattr::data is 2-byte aligned but not 8-byte aligned. */
+		    if (disk_aux) {
+			    x.s[0] = ((__be16 *)disk_aux)[0];
+			    x.s[1] = ((__be16 *)disk_aux)[1];
+			    x.s[2] = ((__be16 *)disk_aux)[2];
+			    x.s[3] = ((__be16 *)disk_aux)[3];
+			    __entry->disk_aux = be64_to_cpu(x.ll);
+		    } else {
+			    __entry->disk_aux = 0;
+		    }
 			   ),
 
 	    TP_printk("o=%08x %s B=%llx c=%u aux=%llx dsk=%llx",