CVE-2014-8121: Do not close NSS files database during iteration [BZ #18007]

Robin Hack discovered Samba would enter an infinite loop processing
certain quota-related requests.  We eventually tracked this down to a
glibc issue.

Running a (simplified) test case under strace shows that /etc/passwd
is continuously opened and closed:

…
open("/etc/passwd", O_RDONLY|O_CLOEXEC) = 3
lseek(3, 0, SEEK_CUR)                   = 0
read(3, "root:x:0:0:root:/root:/bin/bash\n"..., 4096) = 2717
lseek(3, 2717, SEEK_SET)                = 2717
close(3)                                = 0
open("/etc/passwd", O_RDONLY|O_CLOEXEC) = 3
lseek(3, 0, SEEK_CUR)                   = 0
lseek(3, 0, SEEK_SET)                   = 0
read(3, "root:x:0:0:root:/root:/bin/bash\n"..., 4096) = 2717
lseek(3, 2717, SEEK_SET)                = 2717
close(3)                                = 0
open("/etc/passwd", O_RDONLY|O_CLOEXEC) = 3
lseek(3, 0, SEEK_CUR)                   = 0
…

The lookup function implementation in
nss/nss_files/files-XXX.c:DB_LOOKUP has code to prevent that.  It is
supposed skip closing the input file if it was already open.

  /* Reset file pointer to beginning or open file.  */			      \
  status = internal_setent (keep_stream);				      \
									      \
  if (status == NSS_STATUS_SUCCESS)					      \
    {									      \
      /* Tell getent function that we have repositioned the file pointer.  */ \
      last_use = getby;							      \
									      \
      while ((status = internal_getent (result, buffer, buflen, errnop	      \
					H_ERRNO_ARG EXTRA_ARGS_VALUE))	      \
	     == NSS_STATUS_SUCCESS)					      \
	{ break_if_match }						      \
									      \
      if (! keep_stream)						      \
	internal_endent ();						      \
    }									      \

keep_stream is initialized from the stayopen flag in internal_setent.
internal_setent is called from the set*ent implementation as:

  status = internal_setent (stayopen);

However, for non-host database, this flag is always 0, per the
STAYOPEN magic in nss/getXXent_r.c.

Thus, the fix is this:

-  status = internal_setent (stayopen);
+  status = internal_setent (1);

This is not a behavioral change even for the hosts database (where the
application can specify the stayopen flag) because with a call to
sethostent(0), the file handle is still not closed in the
implementation of gethostent.

Conflicts:
	NEWS

1 files changed, 6 insertions, 1 deletions

diff --git a/NEWS b/NEWS
index 8160427710..8a63fb91ca 100644
--- a/NEWS
+++ b/NEWS
@@ -10,7 +10,7 @@ Version 2.20.1
 * The following bugs are resolved with this release:
 
   16009, 16617, 16618, 17266, 17370, 17371, 17460, 17485, 17555, 17625,
-  17630, 17801, 18287
+  17630, 17801, 18007, 18287.
 
 * A buffer overflow in gethostbyname_r and related functions performing DNS
   requests has been fixed.  If the NSS functions were called with a
@@ -19,6 +19,11 @@ Version 2.20.1
   potentially arbitrary code execution, using crafted, but syntactically
   valid DNS responses.  (CVE-2015-1781)
 
+* CVE-2014-8121 The NSS files backend would reset the file pointer used by
+  the get*ent functions if any of the query functions for the same database
+  are used during the iteration, causing a denial-of-service condition in
+  some applications.
+
 * Added support for TSX lock elision of pthread mutexes on powerpc32, powerpc64
   and powerpc64le.  This may improve lock scaling of existing programs on
   HTM capable systems.  The lock elision code is only enabled with