On Sun, 28 Apr 2024 at 17:50, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> But the immediate problem is
> not the user space access, it's that something goes horribly wrong
> *around* it.
Side note: that stack trace from hell actually has three nested page
faults, and I think that's actually the important thing here:
- the first page fault is from user space, and triggers the vsyscall emulation.
- the second page fault is from __do_sys_gettimeofday, and that
should just have caused the exception that then sets the return value
to -EFAULT
- the third nested page fault is due to _raw_spin_unlock_irqrestore
-> preempt_schedule -> trace_sched_switch, which then causes that bpf
trace program to run, which does that bpf_probe_read_compat, which
causes that page fault under pagefault_disable().
It's quite the nasty backtrace, and there's a lot going on.
And I think I finally see what may be going on. The problem is
literally the vsyscall emulation, which sets
current->thread.sig_on_uaccess_err = 1;
and that causes the fixup_exception() code to send the signal
*despite* the exception being caught.
And I think that is in fact completely bogus. It's completely bogus
exactly because it sends that signal even when it *shouldn't* be sent
- like for the bpf user mode trace gathering.
In other words, I think the whole "sig_on_uaccess_err" thing is
entirely broken, because it makes any nested page-faults do all the
wrong things.
Now, arguably, I don't think anybody should enable vsyscall emulation
any more, but this test case clearly does.
I think we should just make the "send SIGSEGV" be something that the
vsyscall emulation does on its own, not this broken per-thread state
for something that isn't actually per thread.
The x86 page fault code actually tried to deal with the "incorrect
nesting" by having that
if (in_interrupt())
return;
which ignores the sig_on_uaccess_err case when it happens in
interrupts, but as shown by this example, these nested page faults do
not need to be about interrupts at all.
IOW, I think the only right thing is to remove that horrendously broken code.
The attached patch is ENTIRELY UNTESTED, but looks like the
ObviouslyCorrect(tm) thing to do.
NOTE! This broken code goes back to commit 4fc3490114bb ("x86-64: Set
siginfo and context on vsyscall emulation faults") in 2011, and back
then the reason was to get all the siginfo details right. Honestly, I
do not for a moment believe that it's worth getting the siginfo
details right here, but part of the commit says
This fixes issues with UML when vsyscall=emulate.
and so my patch to remove this garbage will probably break UML in this
situation.
I cannot find it in myself to care, since I do not believe that
anybody should be running with vsyscall=emulate in 2024 in the first
place, much less if you are doing things like UML. But let's see if
somebody screams.
Also, somebody should obviously test my COMPLETELY UNTESTED patch.
Did I make it clear enough that this is UNTESTED and just does
crapectgomy on something that is clearly broken?
Linus "UNTESTED" Torvalds
arch/x86/entry/vsyscall/vsyscall_64.c | 25 ++-----------------------
arch/x86/include/asm/processor.h | 1 -
arch/x86/mm/fault.c | 33 +--------------------------------
3 files changed, 3 insertions(+), 56 deletions(-)
diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
index a3c0df11d0e6..3b0f61b2ea6d 100644
--- a/arch/x86/entry/vsyscall/vsyscall_64.c
+++ b/arch/x86/entry/vsyscall/vsyscall_64.c
@@ -98,11 +98,6 @@ static int addr_to_vsyscall_nr(unsigned long addr)
static bool write_ok_or_segv(unsigned long ptr, size_t size)
{
- /*
- * XXX: if access_ok, get_user, and put_user handled
- * sig_on_uaccess_err, this could go away.
- */
-
if (!access_ok((void __user *)ptr, size)) {
struct thread_struct *thread = ¤t->thread;
@@ -123,7 +118,6 @@ bool emulate_vsyscall(unsigned long error_code,
struct task_struct *tsk;
unsigned long caller;
int vsyscall_nr, syscall_nr, tmp;
- int prev_sig_on_uaccess_err;
long ret;
unsigned long orig_dx;
@@ -234,12 +228,8 @@ bool emulate_vsyscall(unsigned long error_code,
goto do_ret; /* skip requested */
/*
- * With a real vsyscall, page faults cause SIGSEGV. We want to
- * preserve that behavior to make writing exploits harder.
+ * With a real vsyscall, page faults cause SIGSEGV.
*/
- prev_sig_on_uaccess_err = current->thread.sig_on_uaccess_err;
- current->thread.sig_on_uaccess_err = 1;
-
ret = -EFAULT;
switch (vsyscall_nr) {
case 0:
@@ -262,23 +252,12 @@ bool emulate_vsyscall(unsigned long error_code,
break;
}
- current->thread.sig_on_uaccess_err = prev_sig_on_uaccess_err;
-
check_fault:
if (ret == -EFAULT) {
/* Bad news -- userspace fed a bad pointer to a vsyscall. */
warn_bad_vsyscall(KERN_INFO, regs,
"vsyscall fault (exploit attempt?)");
-
- /*
- * If we failed to generate a signal for any reason,
- * generate one here. (This should be impossible.)
- */
- if (WARN_ON_ONCE(!sigismember(&tsk->pending.signal, SIGBUS) &&
- !sigismember(&tsk->pending.signal, SIGSEGV)))
- goto sigsegv;
-
- return true; /* Don't emulate the ret. */
+ goto sigsegv;
}
regs->ax = ret;
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 811548f131f4..78e51b0d6433 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -472,7 +472,6 @@ struct thread_struct {
unsigned long iopl_emul;
unsigned int iopl_warn:1;
- unsigned int sig_on_uaccess_err:1;
/*
* Protection Keys Register for Userspace. Loaded immediately on
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 622d12ec7f08..bba4e020dd64 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -723,39 +723,8 @@ kernelmode_fixup_or_oops(struct pt_regs *regs, unsigned long error_code,
WARN_ON_ONCE(user_mode(regs));
/* Are we prepared to handle this kernel fault? */
- if (fixup_exception(regs, X86_TRAP_PF, error_code, address)) {
- /*
- * Any interrupt that takes a fault gets the fixup. This makes
- * the below recursive fault logic only apply to a faults from
- * task context.
- */
- if (in_interrupt())
- return;
-
- /*
- * Per the above we're !in_interrupt(), aka. task context.
- *
- * In this case we need to make sure we're not recursively
- * faulting through the emulate_vsyscall() logic.
- */
- if (current->thread.sig_on_uaccess_err && signal) {
- sanitize_error_code(address, &error_code);
-
- set_signal_archinfo(address, error_code);
-
- if (si_code == SEGV_PKUERR) {
- force_sig_pkuerr((void __user *)address, pkey);
- } else {
- /* XXX: hwpoison faults will set the wrong code. */
- force_sig_fault(signal, si_code, (void __user *)address);
- }
- }
-
- /*
- * Barring that, we can do the fixup and be happy.
- */
+ if (fixup_exception(regs, X86_TRAP_PF, error_code, address))
return;
- }
/*
* AMD erratum #91 manifests as a spurious page fault on a PREFETCH
