[PATCH] entry: Fix missed trap after single-step on system call return

From: Gabriel Krisman Bertazi
Date: Wed Feb 03 2021 - 13:03:01 EST

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Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:

> On Sun, Jan 31, 2021 at 3:35 PM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>
>> I wonder if the simple solution is to just
>>
>> (a) always set one of the SYSCALL_WORK_EXIT bits on the child in
>> ptrace (exactly to catch the child on system call exit)
>>
>> (b) basically revert 299155244770 ("entry: Drop usage of TIF flags in
>> the generic syscall code") and have the syscall exit code check the
>> TIF_SINGLESTEP flag
>
> Actually, (b) looks unnecessary - as long as we get to
> syscall_exit_work(), the current code will work fine.
>
> So maybe just add a dummy SYSCALL_WORK_SYSCALL_EXIT_TRAP, and set that
> flag whenever a singestep is requested for a process that is currently
> in a system call?
>
> IOW, make it a very explicit "do TF for system calls", rather than the
> old code that was doing so implicitly and not very obviously. Hmm?

Linus,

Does the patch below follows your suggestion? I'm setting the
SYSCALL_WORK shadowing TIF_SINGLESTEP every time, instead of only when
the child is inside a system call. Is this acceptable?

This seems to pass Kyle's test case. Kyle, can you verify it works with
rr?

I can also turn Kyle's test case into a selftest, if it is ok with him.

Thanks,

-- >8 --
Subject: [PATCH] entry: Fix missed trap after single-step on a system call return

Commit 299155244770 ("entry: Drop usage of TIF flags in the generic
syscall code") introduces a bug on architectures using the generic
syscall entry code, in which processes stopped by PTRACE_SYSCALL do not
trap on syscall return after receiving a TIF_SINGLESTEP. The reason is
the meaning of TIF_SINGLESTEP flag is overloaded to cause the trap after
a system call is executed, but since the above commit, the syscall call
handler only checks for the SYSCALL_WORK flags on the exit work.

This patch splits the meaning of TIF_SINGLESTEP such that it only means
single-step mode, and creates a new type of SYSCALL_WORK to request a
trap immediately after a syscall in single-step mode. In the current
implementation, the SYSCALL_WORK flag shadows the TIF_SINGLESTEP flag
for simplicity.

Since x86 is the only code already using the generic syscall handling,
this also updates that architecture to flip this bit when a tracer
enables single step.

Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Fixes: 299155244770 ("entry: Drop usage of TIF flags in the generic syscall code")
Signed-off-by: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
---
arch/x86/include/asm/entry-common.h | 2 --
arch/x86/kernel/step.c | 10 ++++++++--
include/linux/entry-common.h | 1 +
include/linux/thread_info.h | 2 ++
kernel/entry/common.c | 12 ++----------
5 files changed, 13 insertions(+), 14 deletions(-)

diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index 6fe54b2813c1..2b87b191b3b8 100644
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -43,8 +43,6 @@ static __always_inline void arch_check_user_regs(struct pt_regs *regs)
}
#define arch_check_user_regs arch_check_user_regs

-#define ARCH_SYSCALL_EXIT_WORK (_TIF_SINGLESTEP)
-
static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
unsigned long ti_work)
{
diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index 60d2c3798ba2..de975b62f10a 100644
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -127,12 +127,17 @@ static int enable_single_step(struct task_struct *child)
regs->flags |= X86_EFLAGS_TF;

/*
- * Always set TIF_SINGLESTEP - this guarantees that
- * we single-step system calls etc.. This will also
+ * Always set TIF_SINGLESTEP. This will also
* cause us to set TF when returning to user mode.
*/
set_tsk_thread_flag(child, TIF_SINGLESTEP);

+ /*
+ * Trigger a trap is triggered once stepping out of a system
+ * call prior to executing any user instruction.
+ */
+ set_task_syscall_work(child, SYSCALL_EXIT_TRAP);
+
oflags = regs->flags;

/* Set TF on the kernel stack.. */
@@ -230,6 +235,7 @@ void user_disable_single_step(struct task_struct *child)

/* Always clear TIF_SINGLESTEP... */
clear_tsk_thread_flag(child, TIF_SINGLESTEP);
+ clear_task_syscall_work(child, SYSCALL_EXIT_TRAP);

/* But touch TF only if it was set by us.. */
if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index ca86a00abe86..a104b298019a 100644
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -46,6 +46,7 @@
SYSCALL_WORK_SYSCALL_TRACE | \
SYSCALL_WORK_SYSCALL_AUDIT | \
SYSCALL_WORK_SYSCALL_USER_DISPATCH | \
+ SYSCALL_WORK_SYSCALL_EXIT_TRAP | \
ARCH_SYSCALL_WORK_EXIT)

/*
diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index c8a974cead73..9b2158c69275 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -43,6 +43,7 @@ enum syscall_work_bit {
SYSCALL_WORK_BIT_SYSCALL_EMU,
SYSCALL_WORK_BIT_SYSCALL_AUDIT,
SYSCALL_WORK_BIT_SYSCALL_USER_DISPATCH,
+ SYSCALL_WORK_BIT_SYSCALL_EXIT_TRAP,
};

#define SYSCALL_WORK_SECCOMP BIT(SYSCALL_WORK_BIT_SECCOMP)
@@ -51,6 +52,7 @@ enum syscall_work_bit {
#define SYSCALL_WORK_SYSCALL_EMU BIT(SYSCALL_WORK_BIT_SYSCALL_EMU)
#define SYSCALL_WORK_SYSCALL_AUDIT BIT(SYSCALL_WORK_BIT_SYSCALL_AUDIT)
#define SYSCALL_WORK_SYSCALL_USER_DISPATCH BIT(SYSCALL_WORK_BIT_SYSCALL_USER_DISPATCH)
+#define SYSCALL_WORK_SYSCALL_EXIT_TRAP BIT(SYSCALL_WORK_BIT_SYSCALL_EXIT_TRAP)
#endif

#include <asm/thread_info.h>
diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 6dd82be60df8..f9d491b17b78 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -209,15 +209,9 @@ static void exit_to_user_mode_prepare(struct pt_regs *regs)
lockdep_sys_exit();
}

-#ifndef _TIF_SINGLESTEP
-static inline bool report_single_step(unsigned long work)
-{
- return false;
-}
-#else
/*
* If SYSCALL_EMU is set, then the only reason to report is when
- * TIF_SINGLESTEP is set (i.e. PTRACE_SYSEMU_SINGLESTEP). This syscall
+ * SINGLESTEP is set (i.e. PTRACE_SYSEMU_SINGLESTEP). This syscall
* instruction has been already reported in syscall_enter_from_user_mode().
*/
static inline bool report_single_step(unsigned long work)
@@ -225,10 +219,8 @@ static inline bool report_single_step(unsigned long work)
if (work & SYSCALL_WORK_SYSCALL_EMU)
return false;

- return !!(current_thread_info()->flags & _TIF_SINGLESTEP);
+ return work & SYSCALL_WORK_SYSCALL_EXIT_TRAP;
}
-#endif
-

static void syscall_exit_work(struct pt_regs *regs, unsigned long work)
{
--
2.30.0

Next message: Gustavo Pimentel: "[PATCH v3 12/15] dmaengine: dw-edma: Fix crash on loading/unloading driver"
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