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authorStephen Smalley <sds@tycho.nsa.gov>2017-01-31 11:54:04 -0500
committerPaul Moore <paul@paul-moore.com>2017-02-07 17:31:38 -0500
commita050a570db0190164e7250013214e29a5a9803ee (patch)
tree5c16761c23373d0713ee5e71b771f9167fb54c53 /security/selinux
parentbfc5e3a6af397dcf9c99a6c1872458e7867c4680 (diff)
downloadlinux-a050a570db0190164e7250013214e29a5a9803ee.tar.bz2
selinux: fix off-by-one in setprocattr
SELinux tries to support setting/clearing of /proc/pid/attr attributes from the shell by ignoring terminating newlines and treating an attribute value that begins with a NUL or newline as an attempt to clear the attribute. However, the test for clearing attributes has always been wrong; it has an off-by-one error, and this could further lead to reading past the end of the allocated buffer since commit bb646cdb12e75d82258c2f2e7746d5952d3e321a ("proc_pid_attr_write(): switch to memdup_user()"). Fix the off-by-one error. Even with this fix, setting and clearing /proc/pid/attr attributes from the shell is not straightforward since the interface does not support multiple write() calls (so shells that write the value and newline separately will set and then immediately clear the attribute, requiring use of echo -n to set the attribute), whereas trying to use echo -n "" to clear the attribute causes the shell to skip the write() call altogether since POSIX says that a zero-length write causes no side effects. Thus, one must use echo -n to set and echo without -n to clear, as in the following example: $ echo -n unconfined_u:object_r:user_home_t:s0 > /proc/$$/attr/fscreate $ cat /proc/$$/attr/fscreate unconfined_u:object_r:user_home_t:s0 $ echo "" > /proc/$$/attr/fscreate $ cat /proc/$$/attr/fscreate Note the use of /proc/$$ rather than /proc/self, as otherwise the cat command will read its own attribute value, not that of the shell. There are no users of this facility to my knowledge; possibly we should just get rid of it. UPDATE: Upon further investigation it appears that a local process with the process:setfscreate permission can cause a kernel panic as a result of this bug. This patch fixes CVE-2017-2618. Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov> [PM: added the update about CVE-2017-2618 to the commit description] Cc: stable@vger.kernel.org # 3.5: d6ea83ec6864e Signed-off-by: Paul Moore <paul@paul-moore.com>
Diffstat (limited to 'security/selinux')
-rw-r--r--security/selinux/hooks.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 98a2e92b3168..aa22a5ef5b05 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -5884,7 +5884,7 @@ static int selinux_setprocattr(struct task_struct *p,
return error;
/* Obtain a SID for the context, if one was specified. */
- if (size && str[1] && str[1] != '\n') {
+ if (size && str[0] && str[0] != '\n') {
if (str[size-1] == '\n') {
str[size-1] = 0;
size--;