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Subject[RFC PATCH v2] ptrace: add PTRACE_GET_SYSCALL_INFO request
PTRACE_GET_SYSCALL_INFO lets ptracer obtain details of the syscall
the tracee is blocked in. The request returns meaningful data only
when the tracee is in a syscall-enter-stop or a syscall-exit-stop.

There are two reasons for a special syscall-related ptrace request.

Firstly, with the current ptrace API there are cases when ptracer cannot
retrieve necessary information about syscalls. Some examples include:
* The notorious int-0x80-from-64-bit-task issue. See [1] for details.
In short, if a 64-bit task performs a syscall through int 0x80, its tracer
has no reliable means to find out that the syscall was, in fact,
a compat syscall, and misidentifies it.
* Syscall-enter-stop and syscall-exit-stop look the same for the tracer.
Common practice is to keep track of the sequence of ptrace-stops in order
not to mix the two syscall-stops up. But it is not as simple as it looks;
for example, strace had a (just recently fixed) long-standing bug where
attaching strace to a tracee that is performing the execve system call
led to the tracer identifying the following syscall-exit-stop as
syscall-enter-stop, which messed up all the state tracking.
* Since the introduction of commit 84d77d3f06e7e8dea057d10e8ec77ad71f721be3
("ptrace: Don't allow accessing an undumpable mm"), both PTRACE_PEEKDATA
and process_vm_readv become unavailable when the process dumpable flag
is cleared. On such architectures as ia64 this results in all syscall
arguments being unavailable.

Secondly, ptracers also have to support a lot of arch-specific code for
obtaining information about the tracee. For some architectures, this
requires a ptrace(PTRACE_PEEKUSER, ...) invocation for every syscall
argument and return value.

PTRACE_GET_SYSCALL_INFO returns the following structure:

struct ptrace_syscall_info {
__u8 op; /* 0 for entry, 1 for exit */
__u8 __pad0[7];
union {
struct {
__s32 nr;
__u32 arch;
__u64 instruction_pointer;
__u64 args[6];
} entry_info;
struct {
__s64 rval;
__u8 is_error;
__u8 __pad1[7];
} exit_info;
};
};

The structure was chosen according to [2], except for one change:
a boolean is_error field is added along with rval. This way the tracer
can more reliably distinguish a return value from an error value.

This patch should be applied on top of [3] and [4].

[1] https://lore.kernel.org/lkml/CA+55aFzcSVmdDj9Lh_gdbz1OzHyEm6ZrGPBDAJnywm2LF_eVyg@mail.gmail.com/
[2] https://lore.kernel.org/lkml/CAObL_7GM0n80N7J_DFw_eQyfLyzq+sf4y2AvsCCV88Tb3AwEHA@mail.gmail.com/
[3] https://lore.kernel.org/lkml/20181119210139.GA8360@altlinux.org/
[4] https://lore.kernel.org/lkml/20181120001128.GA11300@altlinux.org/

Co-authored-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Elvira Khabirova <lineprinter@altlinux.org>
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
---
Changes since v1:
* Do not use task->ptrace.
* Replace entry_info.is_compat with entry_info.arch, use syscall_get_arch().
* Use addr argument of sys_ptrace to get expected size of the struct;
return full size of the struct.

include/linux/ptrace.h | 8 ++++++
include/linux/tracehook.h | 9 ++++--
include/uapi/linux/ptrace.h | 20 +++++++++++++
kernel/ptrace.c | 56 +++++++++++++++++++++++++++++++++++++
4 files changed, 90 insertions(+), 3 deletions(-)

diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index 6c2ffed907f5..909930c893d0 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -46,6 +46,14 @@ extern int ptrace_access_vm(struct task_struct *tsk, unsigned long addr,
#define PT_BLOCKSTEP_BIT 30
#define PT_BLOCKSTEP (1<<PT_BLOCKSTEP_BIT)

+/*
+ * These values are used by tracehook_report_syscall_* to store
+ * information about current syscall-stop in task->ptrace_message
+ * for later use by PTRACE_GET_SYSCALL_INFO.
+ */
+#define PT_SYSCALL_IS_ENTERING 0x80000000U
+#define PT_SYSCALL_IS_EXITING 0x90000000U
+
extern long arch_ptrace(struct task_struct *child, long request,
unsigned long addr, unsigned long data);
extern int ptrace_readdata(struct task_struct *tsk, unsigned long src, char __user *dst, int len);
diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
index 40b0b4c1bf7b..24d0e2215ed2 100644
--- a/include/linux/tracehook.h
+++ b/include/linux/tracehook.h
@@ -57,13 +57,15 @@ struct linux_binprm;
/*
* ptrace report for syscall entry and exit looks identical.
*/
-static inline int ptrace_report_syscall(struct pt_regs *regs)
+static inline int ptrace_report_syscall(struct pt_regs *regs,
+ unsigned long message)
{
int ptrace = current->ptrace;

if (!(ptrace & PT_PTRACED))
return 0;

+ current->ptrace_message = message;
ptrace_notify(SIGTRAP | ((ptrace & PT_TRACESYSGOOD) ? 0x80 : 0));

/*
@@ -76,6 +78,7 @@ static inline int ptrace_report_syscall(struct pt_regs *regs)
current->exit_code = 0;
}

+ current->ptrace_message = 0;
return fatal_signal_pending(current);
}

@@ -101,7 +104,7 @@ static inline int ptrace_report_syscall(struct pt_regs *regs)
static inline __must_check int tracehook_report_syscall_entry(
struct pt_regs *regs)
{
- return ptrace_report_syscall(regs);
+ return ptrace_report_syscall(regs, PT_SYSCALL_IS_ENTERING);
}

/**
@@ -126,7 +129,7 @@ static inline void tracehook_report_syscall_exit(struct pt_regs *regs, int step)
if (step)
user_single_step_report(regs);
else
- ptrace_report_syscall(regs);
+ ptrace_report_syscall(regs, PT_SYSCALL_IS_EXITING);
}

/**
diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
index d5a1b8a492b9..3f19a4458309 100644
--- a/include/uapi/linux/ptrace.h
+++ b/include/uapi/linux/ptrace.h
@@ -73,6 +73,26 @@ struct seccomp_metadata {
__u64 flags; /* Output: filter's flags */
};

+#define PTRACE_GET_SYSCALL_INFO 0x420f
+
+struct ptrace_syscall_info {
+ __u8 op; /* 0 for entry, 1 for exit */
+ __u8 __pad0[7];
+ union {
+ struct {
+ __s32 nr;
+ __u32 arch;
+ __u64 instruction_pointer;
+ __u64 args[6];
+ } entry_info;
+ struct {
+ __s64 rval;
+ __u8 is_error;
+ __u8 __pad1[7];
+ } exit_info;
+ };
+};
+
/* Read signals from a shared (process wide) queue */
#define PTRACE_PEEKSIGINFO_SHARED (1 << 0)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 80b34dffdfb9..7c2e92b6c762 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -30,6 +30,10 @@
#include <linux/cn_proc.h>
#include <linux/compat.h>

+#ifdef CONFIG_HAVE_ARCH_TRACEHOOK
+#include <asm/syscall.h> /* For syscall_get_* */
+#endif
+
/*
* Access another process' address space via ptrace.
* Source/target buffer must be kernel space,
@@ -890,6 +894,52 @@ static int ptrace_regset(struct task_struct *task, int req, unsigned int type,
EXPORT_SYMBOL_GPL(task_user_regset_view);
#endif

+#ifdef CONFIG_HAVE_ARCH_TRACEHOOK
+static int ptrace_get_syscall(struct task_struct *child,
+ unsigned long user_size, void __user *datavp)
+{
+ struct ptrace_syscall_info info;
+ struct pt_regs *regs = task_pt_regs(child);
+ unsigned long args[ARRAY_SIZE(info.entry_info.args)];
+ unsigned long actual_size;
+ unsigned long write_size;
+ int i;
+
+ switch (child->ptrace_message) {
+ case PT_SYSCALL_IS_ENTERING:
+ info.op = 0;
+ info.entry_info.arch = syscall_get_arch(child);
+ info.entry_info.nr = syscall_get_nr(child, regs);
+ info.entry_info.instruction_pointer =
+ instruction_pointer(task_pt_regs(child));
+ syscall_get_arguments(child, regs, 0, ARRAY_SIZE(args), args);
+ for (i = 0; i < ARRAY_SIZE(args); i++)
+ info.entry_info.args[i] = args[i];
+ actual_size =
+ offsetofend(struct ptrace_syscall_info, entry_info);
+ break;
+
+ case PT_SYSCALL_IS_EXITING:
+ info.op = 1;
+ info.exit_info.rval = syscall_get_error(child, regs);
+ info.exit_info.is_error = !!info.exit_info.rval;
+ if (!info.exit_info.is_error) {
+ info.exit_info.rval =
+ syscall_get_return_value(child, regs);
+ }
+ actual_size =
+ offsetofend(struct ptrace_syscall_info, exit_info);
+ break;
+
+ default:
+ return -EINVAL;
+ }
+
+ write_size = min(actual_size, user_size);
+ return copy_to_user(datavp, &info, write_size) ? -EFAULT : actual_size;
+}
+#endif
+
int ptrace_request(struct task_struct *child, long request,
unsigned long addr, unsigned long data)
{
@@ -1105,6 +1155,12 @@ int ptrace_request(struct task_struct *child, long request,
ret = seccomp_get_metadata(child, addr, datavp);
break;

+#ifdef CONFIG_HAVE_ARCH_TRACEHOOK
+ case PTRACE_GET_SYSCALL_INFO:
+ ret = ptrace_get_syscall(child, addr, datavp);
+ break;
+#endif
+
default:
break;
}
--
ldv
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