signal: Replace force_fatal_sig with force_exit_sig when in doubt

Recently to prevent issues with SECCOMP_RET_KILL and similar signals
being changed before they are delivered SA_IMMUTABLE was added.

Unfortunately this broke debuggers[1][2] which reasonably expect
to be able to trap synchronous SIGTRAP and SIGSEGV even when
the target process is not configured to handle those signals.

Add force_exit_sig and use it instead of force_fatal_sig where
historically the code has directly called do_exit.  This has the
implementation benefits of going through the signal exit path
(including generating core dumps) without the danger of allowing
userspace to ignore or change these signals.

This avoids userspace regressions as older kernels exited with do_exit
which debuggers also can not intercept.

In the future is should be possible to improve the quality of
implementation of the kernel by changing some of these force_exit_sig
calls to force_fatal_sig.  That can be done where it matters on
a case-by-case basis with careful analysis.

Reported-by: Kyle Huey <me@kylehuey.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
[1] https://lkml.kernel.org/r/CAP045AoMY4xf8aC_4QU_-j7obuEPYgTcnQQP3Yxk=2X90jtpjw@mail.gmail.com
[2] https://lkml.kernel.org/r/20211117150258.GB5403@xsang-OptiPlex-9020
Fixes: a2a2dd8f85de ("signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed")
Fixes: 856705c39f8a ("signal/m68k: Use force_sigsegv(SIGSEGV) in fpsp040_die")
Fixes: fc99908855da ("signal/powerpc: On swapcontext failure force SIGSEGV")
Fixes: 21d81bf71bfe ("signal/s390: Use force_sigsegv in default_trap_handler")
Fixes: e98d91d2e629 ("signal/sparc32: In setup_rt_frame and setup_fram use force_fatal_sig")
Fixes: 908486441621 ("signal/sparc32: Exit with a fatal signal when try_to_clear_window_buffer fails")
Fixes: eed3b85106a0 ("signal/x86: In emulate_vsyscall force a signal instead of calling do_exit")
Fixes: cac0eb1efcc3 ("signal/vm86_32: Properly send SIGSEGV when the vm86 state cannot be saved.")
Fixes: 05466dc15414 ("exit/syscall_user_dispatch: Send ordinary signals on failure")
Link: https://lkml.kernel.org/r/871r3dqfv8.fsf_-_@email.froward.int.ebiederm.org
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Kees Cook <keescook@chromium.org>
Tested-by: Kyle Huey <khuey@kylehuey.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

diff --git a/arch/m68k/kernel/traps.c b/arch/m68k/kernel/traps.c
index 99058a6da956da2f4c10ace2fc45cc6e7ff19cef..34d6458340b0f79097a7e0f17326012a7823c042 100644 (file)
--- a/arch/m68k/kernel/traps.c
+++ b/arch/m68k/kernel/traps.c
@@ -1145,7 +1145,7 @@ asmlinkage void set_esp0(unsigned long ssp)
  */
 asmlinkage void fpsp040_die(void)
 {
-       force_fatal_sig(SIGSEGV);
+       force_exit_sig(SIGSEGV);
 }
 
 #ifdef CONFIG_M68KFPU_EMU

diff --git a/arch/powerpc/kernel/signal_32.c b/arch/powerpc/kernel/signal_32.c
index 00a9c9cd6d421c4fc1536fdb9e527cd9e78a45a6..3e053e2fd6b693bec8075c06573cfb252241fdd1 100644 (file)
--- a/arch/powerpc/kernel/signal_32.c
+++ b/arch/powerpc/kernel/signal_32.c
@@ -1063,7 +1063,7 @@ SYSCALL_DEFINE3(swapcontext, struct ucontext __user *, old_ctx,
         * We kill the task with a SIGSEGV in this situation.
         */
        if (do_setcontext(new_ctx, regs, 0)) {
-               force_fatal_sig(SIGSEGV);
+               force_exit_sig(SIGSEGV);
                return -EFAULT;
        }
 

diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
index ef518535d436878cdc869725789bdaf048f5d3c5..d1e1fc0acbea32822ce3ab910e6f9ad1565019ea 100644 (file)
--- a/arch/powerpc/kernel/signal_64.c
+++ b/arch/powerpc/kernel/signal_64.c
@@ -704,7 +704,7 @@ SYSCALL_DEFINE3(swapcontext, struct ucontext __user *, old_ctx,
         */
 
        if (__get_user_sigset(&set, &new_ctx->uc_sigmask)) {
-               force_fatal_sig(SIGSEGV);
+               force_exit_sig(SIGSEGV);
                return -EFAULT;
        }
        set_current_blocked(&set);
@@ -713,7 +713,7 @@ SYSCALL_DEFINE3(swapcontext, struct ucontext __user *, old_ctx,
                return -EFAULT;
        if (__unsafe_restore_sigcontext(current, NULL, 0, &new_ctx->uc_mcontext)) {
                user_read_access_end();
-               force_fatal_sig(SIGSEGV);
+               force_exit_sig(SIGSEGV);
                return -EFAULT;
        }
        user_read_access_end();

diff --git a/arch/s390/kernel/traps.c b/arch/s390/kernel/traps.c
index 035705c9f23ea0b03a545e94a857075302164105..2b780786fc689f0d2c664efe9a216199b66ec801 100644 (file)
--- a/arch/s390/kernel/traps.c
+++ b/arch/s390/kernel/traps.c
@@ -84,7 +84,7 @@ static void default_trap_handler(struct pt_regs *regs)
 {
        if (user_mode(regs)) {
                report_user_fault(regs, SIGSEGV, 0);
-               force_fatal_sig(SIGSEGV);
+               force_exit_sig(SIGSEGV);
        } else
                die(regs, "Unknown program exception");
 }

diff --git a/arch/sparc/kernel/signal_32.c b/arch/sparc/kernel/signal_32.c
index cd677bc564a7e279f4f40b2a541749f1d7f5c5f9..ffab16369beac82f9fd6cfe243571888f9609a81 100644 (file)
--- a/arch/sparc/kernel/signal_32.c
+++ b/arch/sparc/kernel/signal_32.c
@@ -244,7 +244,7 @@ static int setup_frame(struct ksignal *ksig, struct pt_regs *regs,
                get_sigframe(ksig, regs, sigframe_size);
 
        if (invalid_frame_pointer(sf, sigframe_size)) {
-               force_fatal_sig(SIGILL);
+               force_exit_sig(SIGILL);
                return -EINVAL;
        }
 
@@ -336,7 +336,7 @@ static int setup_rt_frame(struct ksignal *ksig, struct pt_regs *regs,
        sf = (struct rt_signal_frame __user *)
                get_sigframe(ksig, regs, sigframe_size);
        if (invalid_frame_pointer(sf, sigframe_size)) {
-               force_fatal_sig(SIGILL);
+               force_exit_sig(SIGILL);
                return -EINVAL;
        }
 

diff --git a/arch/sparc/kernel/windows.c b/arch/sparc/kernel/windows.c
index bbbd40cc6b282ad7e42ddffe174170ebccd68c7d..8f20862ccc83e77fd01d12cf4856d01455a37564 100644 (file)
--- a/arch/sparc/kernel/windows.c
+++ b/arch/sparc/kernel/windows.c
@@ -122,7 +122,7 @@ void try_to_clear_window_buffer(struct pt_regs *regs, int who)
                if ((sp & 7) ||
                    copy_to_user((char __user *) sp, &tp->reg_window[window],
                                 sizeof(struct reg_window32))) {
-                       force_fatal_sig(SIGILL);
+                       force_exit_sig(SIGILL);
                        return;
                }
        }

diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
index 0b6b277ee050b5084a4e74335c70ee637be5b21d..fd2ee9408e914a20a4b50a6cdb1249e297e913c2 100644 (file)
--- a/arch/x86/entry/vsyscall/vsyscall_64.c
+++ b/arch/x86/entry/vsyscall/vsyscall_64.c
@@ -226,7 +226,7 @@ bool emulate_vsyscall(unsigned long error_code,
        if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
                warn_bad_vsyscall(KERN_DEBUG, regs,
                                  "seccomp tried to change syscall nr or ip");
-               force_fatal_sig(SIGSYS);
+               force_exit_sig(SIGSYS);
                return true;
        }
        regs->orig_ax = -1;

diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
index cce1c89cb7dfd4897c9256ba1d24516303031e96..c21bcd668284259d8f8833205a936106a8010af6 100644 (file)
--- a/arch/x86/kernel/vm86_32.c
+++ b/arch/x86/kernel/vm86_32.c
@@ -160,7 +160,7 @@ Efault_end:
        user_access_end();
 Efault:
        pr_alert("could not access userspace vm86 info\n");
-       force_fatal_sig(SIGSEGV);
+       force_exit_sig(SIGSEGV);
        goto exit_vm86;
 }
 

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 23505394ef7091d36219cd28b2e4ad617226f920..33a50642cf41c04a538817a5c2507baec91c2a4e 100644 (file)
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -352,6 +352,7 @@ extern __must_check bool do_notify_parent(struct task_struct *, int);
 extern void __wake_up_parent(struct task_struct *p, struct task_struct *parent);
 extern void force_sig(int);
 extern void force_fatal_sig(int);
+extern void force_exit_sig(int);
 extern int send_sig(int, struct task_struct *, int);
 extern int zap_other_threads(struct task_struct *p);
 extern struct sigqueue *sigqueue_alloc(void);

diff --git a/kernel/entry/syscall_user_dispatch.c b/kernel/entry/syscall_user_dispatch.c
index 4508201847d22ec7b7f7749693fafc1b8453ba5b..0b6379adff6bdd4327ace5ae7029cfd24369e825 100644 (file)
--- a/kernel/entry/syscall_user_dispatch.c
+++ b/kernel/entry/syscall_user_dispatch.c
@@ -48,7 +48,7 @@ bool syscall_user_dispatch(struct pt_regs *regs)
                 * the selector is loaded by userspace.
                 */
                if (unlikely(__get_user(state, sd->selector))) {
-                       force_fatal_sig(SIGSEGV);
+                       force_exit_sig(SIGSEGV);
                        return true;
                }
 
@@ -56,7 +56,7 @@ bool syscall_user_dispatch(struct pt_regs *regs)
                        return false;
 
                if (state != SYSCALL_DISPATCH_FILTER_BLOCK) {
-                       force_fatal_sig(SIGSYS);
+                       force_exit_sig(SIGSYS);
                        return true;
                }
        }

diff --git a/kernel/signal.c b/kernel/signal.c
index 7815e1bbeddc570fb885e995e4e61668aa422069..a629b11bf3e0d9e5d02ba7341094c8b313ec3244 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1671,6 +1671,19 @@ void force_fatal_sig(int sig)
        force_sig_info_to_task(&info, current, HANDLER_SIG_DFL);
 }
 
+void force_exit_sig(int sig)
+{
+       struct kernel_siginfo info;
+
+       clear_siginfo(&info);
+       info.si_signo = sig;
+       info.si_errno = 0;
+       info.si_code = SI_KERNEL;
+       info.si_pid = 0;
+       info.si_uid = 0;
+       force_sig_info_to_task(&info, current, HANDLER_EXIT);
+}
+
 /*
  * When things go south during signal handling, we
  * will force a SIGSEGV. And if the signal that caused