[PATCH 1/7] smb: client: fix use-after-free infoleak via the readdir resume name

[Diego Oliva <diego@bynar.io>]

CIFSFindNext() copies the resume name that cifs_save_resume_key()
recorded from the last entry of the previous response into its request;
psrch_inf->presume_name points into the response buffer that entry came
from, and nothing ever clears it. find_cifs_entry() records a new
name only from a response with a usable last entry, and cifs_readdir()
only from an entry it walks to, so a response that carries no entries
and leaves last_entry NULL, because CIFSFindFirst() or CIFSFindNext()
rejected LastNameOffset, refreshes neither and presume_name keeps
pointing into the previous response. That buffer has already been
released: by CIFSFindNext() before it installs the new response, or by
find_cifs_entry() when it rewinds the search, which does not drop the
name either and can then fail in initiate_cifs_search() before a new
response replaces it. The next CIFSFindNext() then copies up to
PATH_MAX - 1 bytes of the resume name out of the released buffer into
its request and sends them to the server: a use-after-free read
that leaks kernel memory to a malicious or compromised server, which
can provoke it with a single FindNext response that carries no entries
and a LastNameOffset above CIFSMaxBufSize, which is all the existing
check rejects.

With KASAN enabled, a server that answers a listing that way gives:

 CIFS: VFS: ignoring corrupt resume name
 ==================================================================
 BUG: KASAN: slab-use-after-free in CIFSFindNext+0x8ca/0x14c0
 Read of size 4080 at addr ffff88807c823f98 by task ls/83

 CPU: 0 UID: 0 PID: 83 Comm: ls Tainted: G    B               7.3.0-rc4-00457-gf14572c203d5 #69 PREEMPT(lazy)
 Tainted: [B]=BAD_PAGE
 Call Trace:
  <TASK>
  kasan_report+0xdf/0x1a0
  kasan_check_range+0x10f/0x1e0
  __asan_memcpy+0x23/0x60
  CIFSFindNext+0x8ca/0x14c0
  cifs_readdir+0xf51/0x29c0
  iterate_dir+0x1c0/0x570
  __x64_sys_getdents64+0x133/0x270
  do_syscall_64+0x109/0x5d0
  entry_SYSCALL_64_after_hwframe+0x77/0x7f
  </TASK>

 Allocated by task 83 on cpu 1 at 13.497119s:
  cifs_buf_get+0x36/0x90
  smb_init+0x4f/0x110
  CIFSFindNext+0xf7/0x14c0
  cifs_readdir+0xf51/0x29c0

 Freed by task 83 on cpu 0 at 13.584018s:
  cifs_buf_release+0x41/0x80
  CIFSFindNext+0xfce/0x14c0
  cifs_readdir+0xf51/0x29c0

 The buggy address is located 16280 bytes inside of
  freed 16588-byte region [ffff88807c820000, ffff88807c8240cc)
 ==================================================================

The same memcpy() first reads past the end of that object while it is
still live, a slab-out-of-bounds read that the following patches
bound; that is the taint the report above carries.

SMB1 is not negotiated by default; reaching this code requires an
explicit vers=1.0 mount.

Clear presume_name and resume_name_len whenever a new response is
installed and when the rewind path releases the search buffer, so that
the name never outlives the buffer it points into, and reset
resume_key with them, since it was taken from the same entry and would
otherwise be sent with an empty name. A response without a usable
last entry then continues the search with an empty resume name rather
than a stale one; CIFSFindNext() skips the copy of a zero-length name
so that it never runs on the NULL pointer.

The next patch in the series leaves last_entry NULL for a response
that carries no entries, and bounds LastNameOffset against the
received response rather than against CIFSMaxBufSize. Responses that
are given a last entry today then take the path fixed here, so that
patch must not be applied without this one.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Fixes: b77d753c413e ("[CIFS] Check that last search entry resume key is valid")
Cc: <stable@vger.kernel.org>
Assisted-by: Bynario AI
Signed-off-by: Diego Oliva <diego@bynar.io>
---
 fs/smb/client/cifssmb.c | 9 ++++++++-
 fs/smb/client/readdir.c | 3 +++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/smb/client/cifssmb.c b/fs/smb/client/cifssmb.c
index 6dddbd84b93b..67f033b960e6 100644
--- a/fs/smb/client/cifssmb.c
+++ b/fs/smb/client/cifssmb.c
@@ -4543,6 +4543,9 @@ CIFSFindFirst(const unsigned int xid, struct cifs_tcon *tcon,
 	psrch_inf->entries_in_buffer = le16_to_cpu(parms->SearchCount);
 	psrch_inf->index_of_last_entry = 2 /* skip . and .. */ +
 		psrch_inf->entries_in_buffer;
+	psrch_inf->presume_name = NULL;
+	psrch_inf->resume_name_len = 0;
+	psrch_inf->resume_key = 0;
 	lnoff = le16_to_cpu(parms->LastNameOffset);
 	if (CIFSMaxBufSize < lnoff) {
 		cifs_dbg(VFS, "ignoring corrupt resume name\n");
@@ -4607,7 +4610,8 @@ int CIFSFindNext(const unsigned int xid, struct cifs_tcon *tcon,
 	name_len = psrch_inf->resume_name_len;
 	params += name_len;
 	if (name_len < PATH_MAX) {
-		memcpy(pSMB->ResumeFileName, psrch_inf->presume_name, name_len);
+		if (name_len)
+			memcpy(pSMB->ResumeFileName, psrch_inf->presume_name, name_len);
 		byte_count += name_len;
 		/* 14 byte parm len above enough for 2 byte null terminator */
 		pSMB->ResumeFileName[name_len] = 0;
@@ -4662,6 +4666,9 @@ int CIFSFindNext(const unsigned int xid, struct cifs_tcon *tcon,
 	psrch_inf->endOfSearch = !!parms->EndofSearch;
 	psrch_inf->entries_in_buffer = le16_to_cpu(parms->SearchCount);
 	psrch_inf->index_of_last_entry += psrch_inf->entries_in_buffer;
+	psrch_inf->presume_name = NULL;
+	psrch_inf->resume_name_len = 0;
+	psrch_inf->resume_key = 0;
 	lnoff = le16_to_cpu(parms->LastNameOffset);
 	if (CIFSMaxBufSize < lnoff) {
 		cifs_dbg(VFS, "ignoring corrupt resume name\n");
diff --git a/fs/smb/client/readdir.c b/fs/smb/client/readdir.c
index 9530e5b01564..6ab4e687c3e4 100644
--- a/fs/smb/client/readdir.c
+++ b/fs/smb/client/readdir.c
@@ -752,6 +752,9 @@ find_cifs_entry(const unsigned int xid, struct cifs_tcon *tcon, loff_t pos,
 			cfile->srch_inf.ntwrk_buf_start = NULL;
 			cfile->srch_inf.srch_entries_start = NULL;
 			cfile->srch_inf.last_entry = NULL;
+			cfile->srch_inf.presume_name = NULL;
+			cfile->srch_inf.resume_name_len = 0;
+			cfile->srch_inf.resume_key = 0;
 		}
 		rc = initiate_cifs_search(xid, file, full_path);
 		if (rc) {
-- 
2.39.5