On Tue, Jan 10, 2023 at 9:14 AM Christian Göttsche
<cgzones@xxxxxxxxxxxxxx> wrote:
>
> On Tue, 10 Jan 2023 at 14:18, Stephen Smalley
> <stephen.smalley.work@xxxxxxxxx> wrote:
> >
> > On Mon, Jan 9, 2023 at 12:17 PM Christian Göttsche
> > <cgzones@xxxxxxxxxxxxxx> wrote:
> > >
> > > Add the public interfaces getpidprevcon(3) and getpidprevcon_raw(3), and
> > > the utility getpidprevcon to gather the previous context before the last
> > > exec of a given process.
> >
> > Wondering if we should warn in the manual page for this and other
> > getpid*con() interfaces that they are inherently racy and therefore
> > should never be relied upon for security purposes.
>
> With "and other getpid*con() interfaces" do you mean just getpidcon(3)
> (and getpidcon_raw(3)), which is currently the only interface matching
> your regex and operating on a foreign process, or also getcon(3) and
> getprevcon(3)?
>
> With race conditions you mean the possibility of the target process
> changing its context (via setcon(3)) or querying a replaced process
> (which is mainly possible via spawning a new process after setting
> /proc/sys/kernel/ns_last_pid)?
> Or do you have others in mind?
Yes, just getpidcon/getpidcon_raw. And correct - the inherent race is
with respect to looking up a target process by pid,
when said process may potentially switch its context via setcon or
execve or exit and be replaced by a process of another context.
This was actually successfully done in Android against a service that
was using getpidcon() to obtain the context of the binder client
before support for passing contexts across binder IPC was added.
>
> p.s.:
> after the discussion has settled I'll send a v2 with the interfaces
> actually published in libselinux.map
>
> > >
> > > Signed-off-by: Christian Göttsche <cgzones@xxxxxxxxxxxxxx>
> > > ---
> > > libselinux/include/selinux/selinux.h | 5 ++++
> > > libselinux/man/man3/getcon.3 | 9 +++++++
> > > libselinux/man/man3/getpidprevcon.3 | 1 +
> > > libselinux/man/man3/getpidprevcon_raw.3 | 1 +
> > > libselinux/src/procattr.c | 18 ++++++++++++++
> > > libselinux/utils/.gitignore | 1 +
> > > libselinux/utils/getpidprevcon.c | 33 +++++++++++++++++++++++++
> > > 7 files changed, 68 insertions(+)
> > > create mode 100644 libselinux/man/man3/getpidprevcon.3
> > > create mode 100644 libselinux/man/man3/getpidprevcon_raw.3
> > > create mode 100644 libselinux/utils/getpidprevcon.c
> > >
> > > diff --git a/libselinux/include/selinux/selinux.h b/libselinux/include/selinux/selinux.h
> > > index 47af9953..a0948853 100644
> > > --- a/libselinux/include/selinux/selinux.h
> > > +++ b/libselinux/include/selinux/selinux.h
> > > @@ -54,6 +54,11 @@ extern int getpidcon_raw(pid_t pid, char ** con);
> > > extern int getprevcon(char ** con);
> > > extern int getprevcon_raw(char ** con);
> > >
> > > +/* Get previous context (prior to last exec) of process identified by pid, and
> > > + set *con to refer to it. Caller must free via freecon. */
> > > +extern int getpidprevcon(pid_t pid, char ** con);
> > > +extern int getpidprevcon_raw(pid_t pid, char ** con);
> > > +
> > > /* Get exec context, and set *con to refer to it.
> > > Sets *con to NULL if no exec context has been set, i.e. using default.
> > > If non-NULL, caller must free via freecon. */
> > > diff --git a/libselinux/man/man3/getcon.3 b/libselinux/man/man3/getcon.3
> > > index e7e394f3..38da958b 100644
> > > --- a/libselinux/man/man3/getcon.3
> > > +++ b/libselinux/man/man3/getcon.3
> > > @@ -23,6 +23,10 @@ setcon \- set current security context of a process
> > > .sp
> > > .BI "int getpidcon_raw(pid_t " pid ", char **" context );
> > > .sp
> > > +.BI "int getpidprevcon(pid_t " pid ", char **" context );
> > > +.sp
> > > +.BI "int getpidprevcon_raw(pid_t " pid ", char **" context );
> > > +.sp
> > > .BI "int getpeercon(int " fd ", char **" context );
> > > .sp
> > > .BI "int getpeercon_raw(int " fd ", char **" context );
> > > @@ -50,6 +54,11 @@ same as getcon but gets the context before the last exec.
> > > returns the process context for the specified PID, which must be free'd with
> > > .BR freecon ().
> > >
> > > +.TP
> > > +.BR getpidprevcon ()
> > > +returns the process context before the last exec for the specified PID, which must be free'd with
> > > +.BR freecon ().
> > > +
> > > .TP
> > > .BR getpeercon ()
> > > retrieves the context of the peer socket, which must be free'd with
> > > diff --git a/libselinux/man/man3/getpidprevcon.3 b/libselinux/man/man3/getpidprevcon.3
> > > new file mode 100644
> > > index 00000000..1210b5a0
> > > --- /dev/null
> > > +++ b/libselinux/man/man3/getpidprevcon.3
> > > @@ -0,0 +1 @@
> > > +.so man3/getcon.3
> > > diff --git a/libselinux/man/man3/getpidprevcon_raw.3 b/libselinux/man/man3/getpidprevcon_raw.3
> > > new file mode 100644
> > > index 00000000..1210b5a0
> > > --- /dev/null
> > > +++ b/libselinux/man/man3/getpidprevcon_raw.3
> > > @@ -0,0 +1 @@
> > > +.so man3/getcon.3
> > > diff --git a/libselinux/src/procattr.c b/libselinux/src/procattr.c
> > > index 6f4cfb82..b7a93a2b 100644
> > > --- a/libselinux/src/procattr.c
> > > +++ b/libselinux/src/procattr.c
> > > @@ -300,3 +300,21 @@ int getpidcon(pid_t pid, char **c)
> > > }
> > > return getprocattrcon(c, pid, "current", NULL);
> > > }
> > > +
> > > +int getpidprevcon_raw(pid_t pid, char **c)
> > > +{
> > > + if (pid <= 0) {
> > > + errno = EINVAL;
> > > + return -1;
> > > + }
> > > + return getprocattrcon_raw(c, pid, "prev", NULL);
> > > +}
> > > +
> > > +int getpidprevcon(pid_t pid, char **c)
> > > +{
> > > + if (pid <= 0) {
> > > + errno = EINVAL;
> > > + return -1;
> > > + }
> > > + return getprocattrcon(c, pid, "prev", NULL);
> > > +}
> > > diff --git a/libselinux/utils/.gitignore b/libselinux/utils/.gitignore
> > > index 3ef34374..b19b94a8 100644
> > > --- a/libselinux/utils/.gitignore
> > > +++ b/libselinux/utils/.gitignore
> > > @@ -9,6 +9,7 @@ getdefaultcon
> > > getenforce
> > > getfilecon
> > > getpidcon
> > > +getpidprevcon
> > > getsebool
> > > getseuser
> > > matchpathcon
> > > diff --git a/libselinux/utils/getpidprevcon.c b/libselinux/utils/getpidprevcon.c
> > > new file mode 100644
> > > index 00000000..662ad500
> > > --- /dev/null
> > > +++ b/libselinux/utils/getpidprevcon.c
> > > @@ -0,0 +1,33 @@
> > > +#include <unistd.h>
> > > +#include <stdio.h>
> > > +#include <stdlib.h>
> > > +#include <string.h>
> > > +#include <errno.h>
> > > +#include <selinux/selinux.h>
> > > +
> > > +int main(int argc, char **argv)
> > > +{
> > > + pid_t pid;
> > > + char *buf;
> > > + int rc;
> > > +
> > > + if (argc != 2) {
> > > + fprintf(stderr, "usage: %s pid\n", argv[0]);
> > > + exit(1);
> > > + }
> > > +
> > > + if (sscanf(argv[1], "%d", &pid) != 1) {
> > > + fprintf(stderr, "%s: invalid pid %s\n", argv[0], argv[1]);
> > > + exit(2);
> > > + }
> > > +
> > > + rc = getpidprevcon(pid, &buf);
> > > + if (rc < 0) {
> > > + fprintf(stderr, "%s: getpidprevcon() failed: %s\n", argv[0], strerror(errno));
> > > + exit(3);
> > > + }
> > > +
> > > + printf("%s\n", buf);
> > > + freecon(buf);
> > > + exit(EXIT_SUCCESS);
> > > +}
> > > --
> > > 2.39.0
> > >
