Re: [LTP] [PATCH 0/3] safe_macros: Fix undefined behaviour in vararg handling

[Richard Palethorpe <rpalethorpe@suse.de>]

Hello,

Petr Vorel <pvorel@suse.cz> writes:

>> Hi Tudor,
>
>> > Accessing elements in an empty va_list results in undefined behaviour[0]
>> > that can include accessing arbitrary stack memory. While typically this
>> > doesn't raise a fault, some new more security-oriented architectures
>> > (e.g. CHERI[1] or Morello[2]) don't allow it.
>
>> > Therefore, remove the variadicness from safe_* wrappers that always call
>> > the functions with the optional argument included.
>
>> > Adapt the respective SAFE_* macros to handle the change by passing a
>> > default argument if they're omitted.
>
>> Thanks for an interesting patchset!
>
>> I wonder if it's a correct approach as C API allows

Perhaps, muslc does the following for open()

	if ((flags & O_CREAT) || (flags & O_TMPFILE) == O_TMPFILE) {
		va_list ap;
		va_start(ap, flags);
		mode = va_arg(ap, mode_t);
		va_end(ap);
	}

So it's only accessed if we need the mode. If the error below can be
fixed with the current approach I'd also be happy.


>> int open(const char *pathname, int flags);
>> we will replace it 
>> int open(const char *pathname, int flags, mode_t mode);
>> where mode is 0.
>> But as it's only in safe_* it should be ok.
>> We still have some open() tests without mode, i.e.
>> testcases/kernel/syscalls/open/open06.c
>
>> Unfortunately some tests need to be adjusted, at least all tests in
>> testcases/kernel/syscalls/fgetxattr will fail due int-conversion,
>> as they use NULL as the third parameter:
>
>> $ export CFLAGS="-Werror=conversion"
>> $ ./configure
>> $ cd testcases/kernel/syscalls/fgetxattr
>> $ make clean
>> $ make V=1
>> gcc -I../../../../include -I../../../../include -I../../../../include/old/ -Werror=int-conversion -g -O2 -fno-strict-aliasing -pipe -Wall -W -Wold-style-definition -std=gnu99 -L../../../../lib fgetxattr01.c  -lltp -o fgetxattr01
>> In file included from ../../../../include/tst_test.h:98,
>>                  from fgetxattr01.c:34:
>> fgetxattr01.c: In function ‘setup’:
>> ../../../../include/tst_safe_macros.h:90:67: error: passing argument 6 of ‘safe_open’ makes integer from pointer without a cast [-Werror=int-conversion]
>>    90 |         safe_open(__FILE__, __LINE__, NULL, (pathname), (oflags), (mode))
>>       |                                                                   ^~~~~~
>
>>       |                                                                   void *
>> ../../../../include/tst_safe_macros.h:93:9: note: in expansion of macro ‘__SAFE_OPEN’
>>    93 |         __SAFE_OPEN((pathname), (oflags), ##__VA_ARGS__, 0)
>>       |         ^~~~~~~~~~~
>> fgetxattr01.c:114:14: note: in expansion of macro ‘SAFE_OPEN’
>>   114 |         fd = SAFE_OPEN(FNAME, O_RDONLY, NULL);
>>       |              ^~~~~~~~~
>> In file included from ../../../../include/tst_safe_macros.h:24:
>> ../../../../include/safe_macros_fn.h:78:22: note: expected ‘mode_t’ {aka ‘unsigned int’} but argument is of type ‘void *’
>>    78 |               mode_t mode);
>>       |               ~~~~~~~^~~~
>> cc1: some warnings being treated as errors
>> make: *** [../../../../include/mk/rules.mk:43: fgetxattr01] Error 1
>
> NOTE: this is from gcc, but obviously also clang has the same problem,
> even without -Werror=int-conversion in CFLAGS, obviously it's the default.
> That's why it was found on Fedora, which we test with clang.
>
> Kind regards,
> Petr
>
>> Found on recent openSUSE Tumbleweed which has 2.36. Also our CI has caught it on
>> Fedora (which also uses 2.36):
>> https://github.com/pevik/ltp/actions/runs/3573215390/jobs/6007016572
>
>> Kind regards,
>> Petr
>
>> > [0]: [ISO/IEC 9899:2011] Programming Languages—C, 3rd ed, paragraph 7.16.1.1
>> > [1]: https://www.cl.cam.ac.uk/research/security/ctsrd/cheri/
>> > [2]: https://www.morello-project.org/
>
>> > Tudor Cretu (3):
>> >   safe_open: Fix undefined behaviour in vararg handling
>> >   safe_openat: Fix undefined behaviour in vararg handling
>> >   safe_semctl: Fix undefined behaviour in vararg handling
>
>> >  include/old/safe_macros.h   |  6 ++++--
>> >  include/safe_macros_fn.h    |  3 ++-
>> >  include/tst_safe_file_at.h  | 10 ++++++----
>> >  include/tst_safe_macros.h   |  6 ++++--
>> >  include/tst_safe_sysv_ipc.h | 14 +++++++++-----
>> >  lib/safe_macros.c           | 13 +------------
>> >  lib/tst_cgroup.c            |  2 +-
>> >  lib/tst_safe_file_at.c      | 11 +++--------
>> >  lib/tst_safe_sysv_ipc.c     | 10 +---------
>> >  9 files changed, 31 insertions(+), 44 deletions(-)


-- 
Thank you,
Richard.

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