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seccomp: add SECCOMP_RET_ERRNO 

This change adds the SECCOMP_RET_ERRNO as a valid return value from a
seccomp filter.  Additionally, it makes the first use of the lower
16-bits for storing a filter-supplied errno.  16-bits is more than
enough for the errno-base.h calls.

Returning errors instead of immediately terminating processes that
violate seccomp policy allow for broader use of this functionality
for kernel attack surface reduction.  For example, a linux container
could maintain a whitelist of pre-existing system calls but drop
all new ones with errnos.  This would keep a logically static attack
surface while providing errnos that may allow for graceful failure
without the downside of do_exit() on a bad call.

This change also changes the signature of __secure_computing.  It
appears the only direct caller is the arm entry code and it clobbers
any possible return value (register) immediately.

Signed-off-by: Will Drewry <wad@chromium.org>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Eric Paris <eparis@redhat.com>

v18: - fix up comments and rebase
     - fix bad var name which was fixed in later revs
     - remove _int() and just change the __secure_computing signature
v16-v17: ...
v15: - use audit_seccomp and add a skip label. (eparis@redhat.com)
     - clean up and pad out return codes (indan@nul.nu)
v14: - no change/rebase
v13: - rebase on to 88ebdda615
v12: - move to WARN_ON if filter is NULL
       (oleg@redhat.com, luto@mit.edu, keescook@chromium.org)
     - return immediately for filter==NULL (keescook@chromium.org)
     - change evaluation to only compare the ACTION so that layered
       errnos don't result in the lowest one being returned.
       (keeschook@chromium.org)
v11: - check for NULL filter (keescook@chromium.org)
v10: - change loaders to fn
 v9: - n/a
 v8: - update Kconfig to note new need for syscall_set_return_value.
     - reordered such that TRAP behavior follows on later.
     - made the for loop a little less indent-y
 v7: - introduced
Signed-off-by: James Morris <james.l.morris@oracle.com>
This commit is contained in:
Will Drewry 2012-04-12 16:47:59 -05:00 committed by James Morris
parent 3dc1c1b2d2
commit acf3b2c71e
3 changed files with 42 additions and 16 deletions
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6
arch/Kconfig
View File

@ -220,8 +220,10 @@ config HAVE_ARCH_SECCOMP_FILTER
bool
help
This symbol should be selected by an architecure if it provides
asm/syscall.h, specifically syscall_get_arguments() and
syscall_get_arch().
asm/syscall.h, specifically syscall_get_arguments(),
syscall_get_arch(), and syscall_set_return_value(). Additionally,
its system call entry path must respect a return value of -1 from
__secure_computing() and/or secure_computing().
config SECCOMP_FILTER
def_bool y

10
include/linux/seccomp.h
View File

@ -12,13 +12,14 @@
/*
* All BPF programs must return a 32-bit value.
* The bottom 16-bits are reserved for future use.
* The bottom 16-bits are for optional return data.
* The upper 16-bits are ordered from least permissive values to most.
*
* The ordering ensures that a min_t() over composed return values always
* selects the least permissive choice.
*/
#define SECCOMP_RET_KILL 0x00000000U /* kill the task immediately */
#define SECCOMP_RET_ERRNO 0x00050000U /* returns an errno */
#define SECCOMP_RET_ALLOW 0x7fff0000U /* allow */
/* Masks for the return value sections. */
@ -64,11 +65,12 @@ struct seccomp {
struct seccomp_filter *filter;
};
extern void __secure_computing(int);
static inline void secure_computing(int this_syscall)
extern int __secure_computing(int);
static inline int secure_computing(int this_syscall)
{
if (unlikely(test_thread_flag(TIF_SECCOMP)))
__secure_computing(this_syscall);
return __secure_computing(this_syscall);
return 0;
}
extern long prctl_get_seccomp(void);

42
kernel/seccomp.c
View File

@ -199,15 +199,20 @@ static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen)
static u32 seccomp_run_filters(int syscall)
{
struct seccomp_filter *f;
u32 ret = SECCOMP_RET_KILL;
u32 ret = SECCOMP_RET_ALLOW;
/* Ensure unexpected behavior doesn't result in failing open. */
if (WARN_ON(current->seccomp.filter == NULL))
return SECCOMP_RET_KILL;
/*
* All filters in the list are evaluated and the lowest BPF return
* value always takes priority.
* value always takes priority (ignoring the DATA).
*/
for (f = current->seccomp.filter; f; f = f->prev) {
ret = sk_run_filter(NULL, f->insns);
if (ret != SECCOMP_RET_ALLOW)
break;
u32 cur_ret = sk_run_filter(NULL, f->insns);
if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION))
ret = cur_ret;
}
return ret;
}
@ -346,11 +351,13 @@ static int mode1_syscalls_32[] = {
};
#endif
void __secure_computing(int this_syscall)
int __secure_computing(int this_syscall)
{
int mode = current->seccomp.mode;
int exit_sig = 0;
int *syscall;
u32 ret = SECCOMP_RET_KILL;
int data;
switch (mode) {
case SECCOMP_MODE_STRICT:
@ -361,14 +368,26 @@ void __secure_computing(int this_syscall)
#endif
do {
if (*syscall == this_syscall)
return;
return 0;
} while (*++syscall);
exit_sig = SIGKILL;
break;
#ifdef CONFIG_SECCOMP_FILTER
case SECCOMP_MODE_FILTER:
if (seccomp_run_filters(this_syscall) == SECCOMP_RET_ALLOW)
return;
ret = seccomp_run_filters(this_syscall);
data = ret & SECCOMP_RET_DATA;
switch (ret & SECCOMP_RET_ACTION) {
case SECCOMP_RET_ERRNO:
/* Set the low-order 16-bits as a errno. */
syscall_set_return_value(current, task_pt_regs(current),
-data, 0);
goto skip;
case SECCOMP_RET_ALLOW:
return 0;
case SECCOMP_RET_KILL:
default:
break;
}
exit_sig = SIGSYS;
break;
#endif
@ -379,8 +398,11 @@ void __secure_computing(int this_syscall)
#ifdef SECCOMP_DEBUG
dump_stack();
#endif
audit_seccomp(this_syscall, exit_code, SECCOMP_RET_KILL);
audit_seccomp(this_syscall, exit_sig, ret);
do_exit(exit_sig);
skip:
audit_seccomp(this_syscall, exit_sig, ret);
return -1;
}
long prctl_get_seccomp(void)