ptrace_syscall_32 is failing
Thomas Gleixner
tglx at linutronix.de
Tue Sep 1 19:50:54 EDT 2020
On Sun, Aug 30 2020 at 08:52, Andy Lutomirski wrote:
>> > [RUN] SYSCALL
>> > [FAIL] Initial args are wrong (nr=29, args=0 0 0 0 0 4289172732)
>> > [RUN] SYSCALL
>> > [OK] Args after SIGUSR1 are correct (ax = -514)
>> > [OK] Child got SIGUSR1
>> > [RUN] Step again
>> > [OK] pause(2) restarted correctly
>>
>> Bisected to commit 0b085e68f407 ("x86/entry: Consolidate 32/64 bit
>> syscall entry").
>> It looks like it is because syscall_enter_from_user_mode() is called
>> before reading the 6th argument from the user stack.
Bah.I don't know how I managed to miss that part and interestingly
enough that none of the robots caught that either
> Thomas, can we revert the syscall_enter() and syscall_exit() part of
> the series?
Hrm.
> I think that they almost work for x86, but not quite as
> indicated by this bug. Even if we imagine we can somehow hack around
> this bug, I imagine we're going to find other problems with this
> model, e.g. the potential upcoming exit problem I noted in my review.
What's the upcoming problem?
> I really think the model should be:
>
> void do_syscall_whatever(...)
> {
> irqentry_enter(...);
> instrumentation_begin();
>
> /* Do whatever arch ABI oddities are needed on entry. */
>
> Then either:
> syscall_begin(arch, nr, regs);
> dispatch the syscall;
> syscall_end(arch, nr, regs);
>
> Or just:
> generic_do_syscall(arch, nr, regs);
>
> /* Do whatever arch ABI oddities are needed on exit from the syscall. */
>
> instrumentation_end();
> irqentry_exit(...);
> }
I don't think we want that in general. The current variant is perfectly
fine for everything except the 32bit fast syscall nonsense. Also
irqentry_entry/exit is not equivalent to the syscall_enter/exit
counterparts.
> x86 has an ABI oddity needed on entry: this fast syscall argument
> fixup. We also might end up with ABI oddities on exit if we ever try
> to make single-stepping of syscalls work fully correctly. x86 sort of
> gets away without specifying arch because the arch helpers that get
> called for audit, etc can deduce the arch, but this is kind of gross.
> I suppose we could omit arch as an explicit parameter.
I had that in one of the early versions and was advised to drop that.
> Or I suppose we could try to rejigger the API in time for 5.9.
> Fortunately only x86 uses the new APIs so far. I cc'd a bunch of
> other arch maintainers to see if other architectures fit well in the
> new syscall_enter() model, but I feel like the fact that x86 is
> already broken indicates that we messed it up a bit.
It's not unfixable and the fix is close to what you suggested above
except that it preserves the straight forward stuff for the !32bit fast
syscall case. Completely untested patch below. I run proper tests
tomorrow with brain awake.
Thanks,
tglx
---
arch/x86/entry/common.c | 29 ++++++++++++++++--------
include/linux/entry-common.h | 51 +++++++++++++++++++++++++++++++++++--------
kernel/entry/common.c | 35 ++++++++++++++++++++++++-----
3 files changed, 91 insertions(+), 24 deletions(-)
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -60,16 +60,10 @@
#if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION)
static __always_inline unsigned int syscall_32_enter(struct pt_regs *regs)
{
- unsigned int nr = (unsigned int)regs->orig_ax;
-
if (IS_ENABLED(CONFIG_IA32_EMULATION))
current_thread_info()->status |= TS_COMPAT;
- /*
- * Subtlety here: if ptrace pokes something larger than 2^32-1 into
- * orig_ax, the unsigned int return value truncates it. This may
- * or may not be necessary, but it matches the old asm behavior.
- */
- return (unsigned int)syscall_enter_from_user_mode(regs, nr);
+
+ return (unsigned int)regs->orig_ax;
}
/*
@@ -91,15 +85,29 @@ static __always_inline void do_syscall_3
{
unsigned int nr = syscall_32_enter(regs);
+ /*
+ * Subtlety here: if ptrace pokes something larger than 2^32-1 into
+ * orig_ax, the unsigned int return value truncates it. This may
+ * or may not be necessary, but it matches the old asm behavior.
+ */
+ nr = (unsigned int)syscall_enter_from_user_mode(regs, nr);
+
do_syscall_32_irqs_on(regs, nr);
syscall_exit_to_user_mode(regs);
}
static noinstr bool __do_fast_syscall_32(struct pt_regs *regs)
{
- unsigned int nr = syscall_32_enter(regs);
+ unsigned int nr = syscall_32_enter(regs);
int res;
+ /*
+ * This cannot use syscall_enter_from_user_mode() as it has to
+ * fetch EBP before invoking any of the syscall entry work
+ * functions.
+ */
+ syscall_enter_from_user_mode_prepare(regs);
+
instrumentation_begin();
/* Fetch EBP from where the vDSO stashed it. */
if (IS_ENABLED(CONFIG_X86_64)) {
@@ -122,6 +130,9 @@ static noinstr bool __do_fast_syscall_32
return false;
}
+ /* The case truncates any ptrace induced syscall nr > 2^32 -1 */
+ nr = (unsigned int)syscall_enter_from_user_mode_work(regs, nr);
+
/* Now this is just like a normal syscall. */
do_syscall_32_irqs_on(regs, nr);
syscall_exit_to_user_mode(regs);
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -110,15 +110,30 @@ static inline __must_check int arch_sysc
#endif
/**
- * syscall_enter_from_user_mode - Check and handle work before invoking
- * a syscall
+ * syscall_enter_from_user_mode_prepare - Establish state and enable interrupts
* @regs: Pointer to currents pt_regs
- * @syscall: The syscall number
*
* Invoked from architecture specific syscall entry code with interrupts
* disabled. The calling code has to be non-instrumentable. When the
- * function returns all state is correct and the subsequent functions can be
- * instrumented.
+ * function returns all state is correct, interrupts are enabled and the
+ * subsequent functions can be instrumented.
+ *
+ * This handles lockdep, RCU (context tracking) and tracing state.
+ *
+ * This is invoked when there is extra architecture specific functionality
+ * to be done between establishing state and handling user mode entry work.
+ */
+void syscall_enter_from_user_mode_prepare(struct pt_regs *regs);
+
+/**
+ * syscall_enter_from_user_mode_work - Check and handle work before invoking
+ * a syscall
+ * @regs: Pointer to currents pt_regs
+ * @syscall: The syscall number
+ *
+ * Invoked from architecture specific syscall entry code with interrupts
+ * enabled after invoking syscall_enter_from_user_mode_prepare() and extra
+ * architecture specific work.
*
* Returns: The original or a modified syscall number
*
@@ -127,12 +142,30 @@ static inline __must_check int arch_sysc
* syscall_set_return_value() first. If neither of those are called and -1
* is returned, then the syscall will fail with ENOSYS.
*
- * The following functionality is handled here:
+ * It handles the following work items:
*
- * 1) Establish state (lockdep, RCU (context tracking), tracing)
- * 2) TIF flag dependent invocations of arch_syscall_enter_tracehook(),
+ * 1) TIF flag dependent invocations of arch_syscall_enter_tracehook(),
* __secure_computing(), trace_sys_enter()
- * 3) Invocation of audit_syscall_entry()
+ * 2) Invocation of audit_syscall_entry()
+ */
+long syscall_enter_from_user_mode_work(struct pt_regs *regs, long syscall);
+
+/**
+ * syscall_enter_from_user_mode - Establish state and check and handle work
+ * before invoking a syscall
+ * @regs: Pointer to currents pt_regs
+ * @syscall: The syscall number
+ *
+ * Invoked from architecture specific syscall entry code with interrupts
+ * disabled. The calling code has to be non-instrumentable. When the
+ * function returns all state is correct, interrupts are enabled and the
+ * subsequent functions can be instrumented.
+ *
+ * This is combination of syscall_enter_from_user_mode_prepare() and
+ * syscall_enter_from_user_mode_work().
+ *
+ * Returns: The original or a modified syscall number. See
+ * syscall_enter_from_user_mode_work() for further explanation.
*/
long syscall_enter_from_user_mode(struct pt_regs *regs, long syscall);
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -68,22 +68,45 @@ static long syscall_trace_enter(struct p
return ret ? : syscall;
}
-noinstr long syscall_enter_from_user_mode(struct pt_regs *regs, long syscall)
+static __always_inline long
+__syscall_enter_from_user_work(struct pt_regs *regs, long syscall)
{
unsigned long ti_work;
- enter_from_user_mode(regs);
- instrumentation_begin();
-
- local_irq_enable();
ti_work = READ_ONCE(current_thread_info()->flags);
if (ti_work & SYSCALL_ENTER_WORK)
syscall = syscall_trace_enter(regs, syscall, ti_work);
- instrumentation_end();
return syscall;
}
+long syscall_enter_from_user_mode_work(struct pt_regs *regs, long syscall)
+{
+ return __syscall_enter_from_user_work(regs, syscall);
+}
+
+noinstr long syscall_enter_from_user_mode(struct pt_regs *regs, long syscall)
+{
+ long ret;
+
+ enter_from_user_mode(regs);
+
+ instrumentation_begin();
+ local_irq_enable();
+ ret = __syscall_enter_from_user_work(regs, syscall);
+ instrumentation_end();
+
+ return ret;
+}
+
+noinstr void syscall_enter_from_user_mode_prepare(struct pt_regs *regs)
+{
+ enter_from_user_mode(regs);
+ instrumentation_begin();
+ local_irq_enable();
+ instrumentation_end();
+}
+
/**
* exit_to_user_mode - Fixup state when exiting to user mode
*
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