uml: kill processes instead of panicing kernel

UML was panicing in the case of failures of libc calls which shouldn't happen.
 This is an overreaction since a failure from libc doesn't normally mean that
kernel data structures are in an unknown state.  Instead, the current process
should just be killed if there is no way to recover.

The case that prompted this was a failure of PTRACE_SETREGS restoring the same
state that was read by PTRACE_GETREGS.  It appears that when a process tries
to load a bogus value into a segment register, it segfaults (as expected) and
the value is actually loaded and is seen by PTRACE_GETREGS (not expected).

This case is fixed by forcing a fatal SIGSEGV on the process so that it
immediately dies.  fatal_sigsegv was added for this purpose.  It was declared
as noreturn, so in order to pursuade gcc that it actually does not return, I
added a call to os_dump_core (and declared it noreturn) so that I get a core
file if somehow the process survives.

All other calls in arch/um/os-Linux/skas/process.c got the same treatment,
with failures causing the process to die instead of a kernel panic, with some
exceptions.

userspace_tramp exits with status 1 if anything goes wrong there.  That will
cause start_userspace to return an error.  copy_context_skas0 and
map_stub_pages also now return errors instead of panicing.  Callers of thes
functions were changed to check for errors and do something appropriate.
Usually that's to return an error to their callers.
check_skas3_ptrace_faultinfo just exits since that's too early to do anything
else.

save_registers, restore_registers, and init_registers now return status
instead of panicing on failure, with their callers doing something
appropriate.

There were also duplicate declarations of save_registers and restore_registers
in os.h - these are gone.

I noticed and fixed up some whitespace damage.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 33 insertions, 4 deletions

diff --git a/arch/um/kernel/skas/mmu.c b/arch/um/kernel/skas/mmu.c
index b56fe8b67a81..6da9ab4f5a18 100644
--- a/arch/um/kernel/skas/mmu.c
+++ b/arch/um/kernel/skas/mmu.c
@@ -114,6 +114,11 @@ int init_new_context(struct task_struct *task, struct mm_struct *mm)
                         to_mm->id.u.pid = copy_context_skas0(stack,
                                                              from_mm->id.u.pid);
                 else to_mm->id.u.pid = start_userspace(stack);
+
+                if (to_mm->id.u.pid < 0) {
+                        ret = to_mm->id.u.pid;
+                        goto out_free;
+                }
         }
 
         ret = init_new_ldt(to_mm, from_mm);
diff --git a/arch/um/kernel/skas/process.c b/arch/um/kernel/skas/process.c
index fce389c2342f..2e9852c0d487 100644
--- a/arch/um/kernel/skas/process.c
+++ b/arch/um/kernel/skas/process.c
@@ -6,19 +6,25 @@
 #include "linux/init.h"
 #include "linux/sched.h"
 #include "as-layout.h"
+#include "kern.h"
 #include "os.h"
 #include "skas.h"
 
 int new_mm(unsigned long stack)
 {
-        int fd;
+        int fd, err;
 
         fd = os_open_file("/proc/mm", of_cloexec(of_write(OPENFLAGS())), 0);
         if (fd < 0)
                 return fd;
 
-        if (skas_needs_stub)
-                map_stub_pages(fd, STUB_CODE, STUB_DATA, stack);
+        if (skas_needs_stub) {
+                err = map_stub_pages(fd, STUB_CODE, STUB_DATA, stack);
+                if (err) {
+                        os_close_file(fd);
+                        return err;
+                }
+        }
 
         return fd;
 }
@@ -49,8 +55,14 @@ int __init start_uml(void)
 {
         stack_protections((unsigned long) &cpu0_irqstack);
         set_sigstack(cpu0_irqstack, THREAD_SIZE);
-        if (proc_mm)
+        if (proc_mm) {
                 userspace_pid[0] = start_userspace(0);
+                if (userspace_pid[0] < 0) {
+                        printf("start_uml - start_userspace returned %d\n",
+                               userspace_pid[0]);
+                        exit(1);
+                }
+        }
 
         init_new_thread_signals();
 
diff --git a/arch/um/kernel/trap.c b/arch/um/kernel/trap.c
index 8fd1a797c3eb..44e490419495 100644
--- a/arch/um/kernel/trap.c
+++ b/arch/um/kernel/trap.c
@@ -129,6 +129,18 @@ static void bad_segv(struct faultinfo fi, unsigned long ip)
         force_sig_info(SIGSEGV, &si, current);
 }
 
+void fatal_sigsegv(void)
+{
+        force_sigsegv(SIGSEGV, current);
+        do_signal();
+        /*
+         * This is to tell gcc that we're not returning - do_signal
+         * can, in general, return, but in this case, it's not, since
+         * we just got a fatal SIGSEGV queued.
+         */
+        os_dump_core();
+}
+
 void segv_handler(int sig, struct uml_pt_regs *regs)
 {
         struct faultinfo * fi = UPT_FAULTINFO(regs);