Headline
CVE-2021-4209: Null pointer dereference in MD_UPDATE (#1306) · Issues · gnutls / GnuTLS · GitLab
A NULL pointer dereference flaw was found in GnuTLS. As Nettle’s hash update functions internally call memcpy, providing zero-length input may cause undefined behavior. This flaw leads to a denial of service after authentication in rare circumstances.
Description of problem:
Using gnutls with guile disabled, null pointer may passed to memcpy as argument 2, causing null pointer dereference.
How to reproduce:
You can easily reproduce this issue:
Build gnutls with UBSan:
CC=clang CXX=clang++ CFLAGS="-fsanitize=undefined -g" CXXFLAGS="-fsanitize=undefined -g" ./bootstrap CC=clang CXX=clang++ CFLAGS="-fsanitize=undefined -g" CXXFLAGS="-fsanitize=undefined -g" ./configure --disable-guile --disable-doc CC=clang CXX=clang++ CFLAGS="-fsanitize=undefined -g" CXXFLAGS="-fsanitize=undefined -g" make
Run server:
./gnutls/src/.libs/gnutls-serv -p 7834 -d 9999
Run client:
$ UBSAN_OPTIONS=print_stacktrace=1 LD_LIBRARY_PATH="…/…/lib/.libs:/usr/lib64" ./gnutls-cli -p 7834 localhost --pskusername psk_identity --pskkey 88f3824b3e5659f52d00e959bacab954b6540344 --priority NORMAL:-KX-ALL:+ECDHE-PSK:+DHE-PSK:+PSK
Then the program will crash with the following log:
Processed 128 CA certificate(s).
Resolving 'localhost:7834'...
Connecting to '127.0.0.1:7834'...
sha256.c:100:3: runtime error: null pointer passed as argument 2, which is declared to never be null
/usr/include/string.h:44:28: note: nonnull attribute specified here
#0 0x7f9a7c53f146 in nettle_sha256_update /root/projects/bleem/nettle-3.6/sha256.c:100:3
#1 0x7f9a7d935f7d in wrap_nettle_hash_fast /root/projects/bleem/gnutls/lib/nettle/mac.c:791:2
#2 0x7f9a7d2bc1fd in _gnutls_hash_fast /root/projects/bleem/gnutls/lib/hash_int.c:141:8
#3 0x7f9a7d3b8faa in gnutls_hash_fast /root/projects/bleem/gnutls/lib/crypto-api.c:690:9
#4 0x7f9a7d455ee2 in _tls13_derive_secret2 /root/projects/bleem/gnutls/lib/secrets.c:98:8
#5 0x7f9a7d7ed3d0 in compute_binder_key /root/projects/bleem/gnutls/lib/ext/pre_shared_key.c:86:8
#6 0x7f9a7d7ebec1 in compute_psk_binder /root/projects/bleem/gnutls/lib/ext/pre_shared_key.c:170:8
#7 0x7f9a7d7f18af in client_send_params /root/projects/bleem/gnutls/lib/ext/pre_shared_key.c:486:9
#8 0x7f9a7d7e4820 in _gnutls_psk_send_params /root/projects/bleem/gnutls/lib/ext/pre_shared_key.c:797:10
#9 0x7f9a7d2df26b in hello_ext_send /root/projects/bleem/gnutls/lib/hello_ext.c:369:8
#10 0x7f9a7d45aa69 in _gnutls_extv_append /root/projects/bleem/gnutls/lib/extv.c:218:8
#11 0x7f9a7d2dd41a in _gnutls_gen_hello_extensions /root/projects/bleem/gnutls/lib/hello_ext.c:437:9
#12 0x7f9a7d261e96 in send_client_hello /root/projects/bleem/gnutls/lib/handshake.c:2342:8
#13 0x7f9a7d23c6db in handshake_client /root/projects/bleem/gnutls/lib/handshake.c:3043:9
#14 0x7f9a7d23b47f in gnutls_handshake /root/projects/bleem/gnutls/lib/handshake.c:2873:10
#15 0x4df28d in do_handshake /root/projects/bleem/gnutls/src/cli.c:1837:9
#16 0x4ff797 in socket_open2 /root/projects/bleem/gnutls/src/socket.c:602:10
#17 0x4d3f43 in main /root/projects/bleem/gnutls/src/cli.c:1363:2
#18 0x7f9a7ca310b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16
#19 0x42127d in _start (/root/projects/bleem/gnutls/src/.libs/gnutls-cli+0x42127d)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior sha256.c:100:3 in
*** Fatal error: A TLS fatal alert has been received.
*** Received alert [40]: Handshake failed
Here is the debug information:
$ UBSAN_OPTIONS=print_stacktrace=1 LD_LIBRARY_PATH="./gnutls/lib/.libs:/usr/lib64" gdb --args ./gnutls-cli -p 7834 localhost --pskusername psk_identity --pskkey 88f3824b3e5659f52d00e959bacab954b6540344 --priority NORMAL:-KX-ALL:+ECDHE-PSK:+DHE-PSK:+PSK 127.0.0.1
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Reading symbols from ./gnutls-cli...
(gdb) b /root/projects/bleem/nettle-3.6/sha256.c:100
No source file named /root/projects/bleem/nettle-3.6/sha256.c.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (/root/projects/bleem/nettle-3.6/sha256.c:100) pending.
(gdb) r
Starting program: /root/projects/bleem/gnutls/src/.libs/gnutls-cli -p 7834 localhost --pskusername psk_identity --pskkey 88f3824b3e5659f52d00e959bacab954b6540344 --priority NORMAL:-KX-ALL:+ECDHE-PSK:+DHE-PSK:+PSK 127.0.0.1
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Processed 128 CA certificate(s).
Resolving 'localhost:7834'...
Connecting to '127.0.0.1:7834'...
Breakpoint 1, nettle_sha256_update (ctx=0x7fffffff5620, length=64,
data=0x7fffffff52a0 '\\' <repeats 64 times>, "\301\315\001\202\214q\250") at sha256.c:100
100 sha256.c
(gdb) c
Continuing.
Breakpoint 1, nettle_sha256_update (ctx=0x7fffffff5690, length=64,
data=0x7fffffff52a0 '6' <repeats 64 times>, "\301\315\001\202\214q\250") at sha256.c:100
100 in sha256.c
(gdb)
Continuing.
Breakpoint 1, nettle_sha256_update (ctx=0x7fffffff5700, length=20, data=0x603000029830 "\210\363\202K>VY\365-")
at sha256.c:100
100 in sha256.c
(gdb)
Continuing.
Breakpoint 1, nettle_sha256_update (ctx=0x7fffffff5700, length=32,
data=0x7fffffff53c0 "\224(\204\365\362W\002\331\327+\200\250\257\234\017\256m8(q5\267\200x\236\332<\023\004\305\067X") at sha256.c:100
100 in sha256.c
(gdb)
Continuing.
Breakpoint 1, nettle_sha256_update (ctx=0x7fffffff6720, length=0, data=0x0) at sha256.c:100
100 in sha256.c
(gdb)
Continuing.
sha256.c:100:3: runtime error: null pointer passed as argument 2, which is declared to never be null
/usr/include/string.h:44:28: note: nonnull attribute specified here
[Detaching after fork from child process 3176009]
#0 0x7ffff6799146 in nettle_sha256_update /root/projects/bleem/nettle-3.6/sha256.c:100:3
#1 0x7ffff7b8ff7d in wrap_nettle_hash_fast /root/projects/bleem/gnutls/lib/nettle/mac.c:791:2
#2 0x7ffff75161fd in _gnutls_hash_fast /root/projects/bleem/gnutls/lib/hash_int.c:141:8
#3 0x7ffff7612faa in gnutls_hash_fast /root/projects/bleem/gnutls/lib/crypto-api.c:690:9
#4 0x7ffff76afee2 in _tls13_derive_secret2 /root/projects/bleem/gnutls/lib/secrets.c:98:8
#5 0x7ffff7a473d0 in compute_binder_key /root/projects/bleem/gnutls/lib/ext/pre_shared_key.c:86:8
#6 0x7ffff7a45ec1 in compute_psk_binder /root/projects/bleem/gnutls/lib/ext/pre_shared_key.c:170:8
#7 0x7ffff7a4b8af in client_send_params /root/projects/bleem/gnutls/lib/ext/pre_shared_key.c:486:9
#8 0x7ffff7a3e820 in _gnutls_psk_send_params /root/projects/bleem/gnutls/lib/ext/pre_shared_key.c:797:10
#9 0x7ffff753926b in hello_ext_send /root/projects/bleem/gnutls/lib/hello_ext.c:369:8
#10 0x7ffff76b4a69 in _gnutls_extv_append /root/projects/bleem/gnutls/lib/extv.c:218:8
#11 0x7ffff753741a in _gnutls_gen_hello_extensions /root/projects/bleem/gnutls/lib/hello_ext.c:437:9
#12 0x7ffff74bbe96 in send_client_hello /root/projects/bleem/gnutls/lib/handshake.c:2342:8
#13 0x7ffff74966db in handshake_client /root/projects/bleem/gnutls/lib/handshake.c:3043:9
#14 0x7ffff749547f in gnutls_handshake /root/projects/bleem/gnutls/lib/handshake.c:2873:10
#15 0x4df28d in do_handshake /root/projects/bleem/gnutls/src/cli.c:1837:9
#16 0x4ff797 in socket_open2 /root/projects/bleem/gnutls/src/socket.c:602:10
#17 0x4d3f43 in main /root/projects/bleem/gnutls/src/cli.c:1363:2
#18 0x7ffff6c8b0b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16
#19 0x42127d in _start (/root/projects/bleem/gnutls/src/.libs/gnutls-cli+0x42127d)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior sha256.c:100:3 in
Breakpoint 1, nettle_sha256_update (ctx=0x7fffffff5240, length=64,
data=0x7fffffff4ec0 "\310:\216\341j\177\224\020ݚqs\225\372\tV\230\357\242$*\271\003\026\\\234nȓ\343]\231", '\\' <repeats 32 times>, "\001") at sha256.c:100
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