This is a note to let you know that I've just added the patch titled 9p: set req refcount to zero to avoid uninitialized usage to the 4.19-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: 9p-set-req-refcount-to-zero-to-avoid-uninitialized-usage.patch and it can be found in the queue-4.19 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 26273ade77f54716e30dfd40ac6e85ceb54ac0f9 Mon Sep 17 00:00:00 2001 From: Schspa Shi <schspa@xxxxxxxxx> Date: Thu, 1 Dec 2022 11:33:10 +0800 Subject: 9p: set req refcount to zero to avoid uninitialized usage From: Schspa Shi <schspa@xxxxxxxxx> commit 26273ade77f54716e30dfd40ac6e85ceb54ac0f9 upstream. When a new request is allocated, the refcount will be zero if it is reused, but if the request is newly allocated from slab, it is not fully initialized before being added to idr. If the p9_read_work got a response before the refcount initiated. It will use a uninitialized req, which will result in a bad request data struct. Here is the logs from syzbot. Corrupted memory at 0xffff88807eade00b [ 0xff 0x07 0x00 0x00 0x00 0x00 0x00 0x00 . . . . . . . . ] (in kfence-#110): p9_fcall_fini net/9p/client.c:248 [inline] p9_req_put net/9p/client.c:396 [inline] p9_req_put+0x208/0x250 net/9p/client.c:390 p9_client_walk+0x247/0x540 net/9p/client.c:1165 clone_fid fs/9p/fid.h:21 [inline] v9fs_fid_xattr_set+0xe4/0x2b0 fs/9p/xattr.c:118 v9fs_xattr_set fs/9p/xattr.c:100 [inline] v9fs_xattr_handler_set+0x6f/0x120 fs/9p/xattr.c:159 __vfs_setxattr+0x119/0x180 fs/xattr.c:182 __vfs_setxattr_noperm+0x129/0x5f0 fs/xattr.c:216 __vfs_setxattr_locked+0x1d3/0x260 fs/xattr.c:277 vfs_setxattr+0x143/0x340 fs/xattr.c:309 setxattr+0x146/0x160 fs/xattr.c:617 path_setxattr+0x197/0x1c0 fs/xattr.c:636 __do_sys_setxattr fs/xattr.c:652 [inline] __se_sys_setxattr fs/xattr.c:648 [inline] __ia32_sys_setxattr+0xc0/0x160 fs/xattr.c:648 do_syscall_32_irqs_on arch/x86/entry/common.c:112 [inline] __do_fast_syscall_32+0x65/0xf0 arch/x86/entry/common.c:178 do_fast_syscall_32+0x33/0x70 arch/x86/entry/common.c:203 entry_SYSENTER_compat_after_hwframe+0x70/0x82 Below is a similar scenario, the scenario in the syzbot log looks more complicated than this one, but this patch can fix it. T21124 p9_read_work ======================== second trans ================================= p9_client_walk p9_client_rpc p9_client_prepare_req p9_tag_alloc req = kmem_cache_alloc(p9_req_cache, GFP_NOFS); tag = idr_alloc << preempted >> req->tc.tag = tag; /* req->[refcount/tag] == uninitialized */ m->rreq = p9_tag_lookup(m->client, m->rc.tag); /* increments uninitalized refcount */ refcount_set(&req->refcount, 2); /* cb drops one ref */ p9_client_cb(req) /* reader thread drops its ref: request is incorrectly freed */ p9_req_put(req) /* use after free and ref underflow */ p9_req_put(req) To fix it, we can initialize the refcount to zero before add to idr. Link: https://lkml.kernel.org/r/20221201033310.18589-1-schspa@xxxxxxxxx Cc: stable@xxxxxxxxxxxxxxx # 6.0+ due to 6cda12864cb0 ("9p: Drop kref usage") Fixes: 728356dedeff ("9p: Add refcount to p9_req_t") Reported-by: syzbot+8f1060e2aaf8ca55220b@xxxxxxxxxxxxxxxxxxxxxxxxx Signed-off-by: Schspa Shi <schspa@xxxxxxxxx> Reviewed-by: Christian Schoenebeck <linux_oss@xxxxxxxxxxxxx> Signed-off-by: Dominique Martinet <asmadeus@xxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- net/9p/client.c | 5 +++++ 1 file changed, 5 insertions(+) --- a/net/9p/client.c +++ b/net/9p/client.c @@ -298,6 +298,11 @@ p9_tag_alloc(struct p9_client *c, int8_t p9pdu_reset(&req->rc); req->t_err = 0; req->status = REQ_STATUS_ALLOC; + /* refcount needs to be set to 0 before inserting into the idr + * so p9_tag_lookup does not accept a request that is not fully + * initialized. refcount_set to 2 below will mark request ready. + */ + refcount_set(&req->refcount, 0); init_waitqueue_head(&req->wq); INIT_LIST_HEAD(&req->req_list); Patches currently in stable-queue which might be from schspa@xxxxxxxxx are queue-4.19/mrp-introduce-active-flags-to-prevent-uaf-when-appli.patch queue-4.19/9p-set-req-refcount-to-zero-to-avoid-uninitialized-usage.patch