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❌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❌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.
In bug 14906 the user complains that the inotify support in nscd
is not sufficient when it comes to detecting changes in the
configurationfiles that should be watched for the various databases.
The current nscd implementation uses inotify to watch for changes in
the configuration files, but adds watches only for IN_DELETE_SELF and
IN_MODIFY. These watches are insufficient to cover even the most basic
uses by a system administrator. For example using emacs or vim to edit
a configuration file should trigger a reload but it might not if
the editors use move to atomically update the file. This atomic update
changes the inode and thus removes the notification on the file (as
inotify is based on inodes). Thus the inotify support in nscd for
configuration files is insufficient to account for the average use
cases of system administrators and users.
The inotify support is significantly enhanced and described here:
https://www.sourceware.org/ml/libc-alpha/2015-02/msg00504.html
Tested on x86_64 with and without inotify support.
Replace it with IS_IN (libc) and remove the one place that it
is defined in. The generated code remains unchanged on x86_64.
* include/shlib-compat.h [!NOT_IN_libc]: Remove.
* nss/nss_files/files-parse.c (IS_IN_libc): Replace with
IS_IN (libc).
nss_db uses nss_files code for services, but a continue on protocol
mismatch that doesn't affect nss_files skipped the code that advanced
to the next db entry. Any one of these changes would suffice to fix
it, but fixing both makes them both safer to reuse elsewhere.
for ChangeLog
[BZ #14498]
* NEWS: Fixed.
* nss/nss_db/db-XXX.c (_nss_db_get##name##_r): Update hidx
after parsing line but before break_if_match.
* nss/nss_files/files-service (DB_LOOKUP): Don't "continue;"
if there is a protocol mismatch.
The netgroups nss modules in the glibc tree use NSS_STATUS_UNAVAIL
(with errno as ERANGE) when the supplied buffer does not have
sufficient space for the result. This is wrong, because the canonical
way to indicate insufficient buffer is to set the errno to ERANGE and
the status to NSS_STATUS_TRYAGAIN, as is used by all other modules.
This fixes nscd behaviour when the nss_ldap module returns
NSS_STATUS_TRYAGAIN to indicate that a netgroup entry is too long to
fit into the supplied buffer.
The netgroups file parsing code tries to access the character before
the newline in parsed lines to see if it is a backslash (\). This
results in an access before the block allocated for the line if the
line is blank, i.e. does not have anything other than the newline
character. This doesn't seem like it will cause any crashes because
the byte belongs to the malloc metadata block and hence access to it
will always succeed.
There could be an invalid alteration in code flow where a blank line
is seen as a continuation due to the preceding byte *happening* to be
'\\'. This could be done by interposing malloc, but that's not really
a security problem since one could interpose getnetgrent_r itself and
achieve a similar 'exploit'.
The possibility of actually exploiting this is remote to impossible
since it also requires the previous line to end with a '\\', which
would happen only on invalid configurations.
AF_INET lookup in hosts file uses _nss_files_gethostbyname2_r, which
is not capable of returning a canonical name if it has found one.
This change adds _nss_files_gethostbyname3_r, which wraps around
_nss_files_gethostbyname2_r and then returns result.h_name as the
canonical name.
Currently for AF_INET lookups from the hosts file, buffer sizes larger
than INT_MAX silently overflow and may result in access beyond bounds
of a buffer. This happens when the number of results in an AF_INET
lookup in /etc/hosts are very large.
There are two aspects to the problem. One problem is that the size
computed from the buffer size is stored into an int, which results in
overflow for large sizes. Additionally, even if this size was
expanded, the function used to read content into the buffer (fgets)
accepts only int sizes. As a result, the fix is to have a function
wrap around fgets that calls it multiple times with int sizes if
necessary.
nscd can clear caches when certain files change. The list of files
was hardcoded so far and worked for nss_files and nss_dns and those
modules which need no monitoring. nss_db, for instance, has its
own set of files to monitor. Now the NSS modules themselves can
request that certain files are monitored.
No longer is Berkeley db used. Instead a simple hash function is used.
The database files are not updated once they are created and therefore
no complicated database is needed.
