Re: [RFC][PATCH] x86: ptrace: Add function argument access API

[Steven Rostedt]

On Fri, 12 Oct 2018 11:21:28 -0700
Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:

> On Fri, Oct 12, 2018 at 9:26 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> >
> >
> > Anyone have any issues with this patch?
> >  
> 
> I'm conceptually okay with it.  That being said,
> regs_within_kernel_stack(), which you're indirectly using, is
> off-by-a-few.  And updating it to use probe_kernel_read() might be
> nice for robustness.
> 

Something like this?

-- Steve

From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
Date: Fri, 12 Oct 2018 15:44:20 -0400
Subject: [PATCH] x86: ptrace.h: Add regs_get_kernel_stack_nth_safe() function

Andy had some concerns about using regs_get_kernel_stack_nth() in a new
function regs_get_kernel_argument() as if there's any error in the stack
code, it could cause a bad memory access. Instead, add a new function called
regs_get_kernel_stack_nth_safe() that does a probe_kernel_read() on the
stack address to be extra careful in accessing the memory. To share the
code, regs_get_kernel_stack_nth_addr() was added to just return the stack
address (or NULL if not on the stack), that both regs_get_kernel_stack_nth()
and the _safe() version can use.

Link: http://lkml.kernel.org/r/CALCETrXn9zKTb9i1LP3qoFcpqZHF34BdkuZ5D3N0uCmRr+VnbA@xxxxxxxxxxxxxx
Requested-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
 arch/x86/include/asm/ptrace.h | 57 ++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 54 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
index c2304b25e2fd..8df7ab6a17c5 100644
--- a/arch/x86/include/asm/ptrace.h
+++ b/arch/x86/include/asm/ptrace.h
@@ -237,6 +237,27 @@ static inline int regs_within_kernel_stack(struct pt_regs *regs,
 }
 
 /**
+ * regs_get_kernel_stack_nth_addr() - get the address of the Nth entry on stack
+ * @regs:	pt_regs which contains kernel stack pointer.
+ * @n:		stack entry number.
+ *
+ * regs_get_kernel_stack_nth() returns the address of the @n th entry of the
+ * kernel stack which is specified by @regs. If the @n th entry is NOT in
+ * the kernel stack, this returns NULL.
+ */
+static inline unsigned long *regs_get_kernel_stack_nth_addr(struct pt_regs *regs,
+							    unsigned int n)
+{
+	unsigned long *addr = (unsigned long *)kernel_stack_pointer(regs);
+
+	addr += n;
+	if (regs_within_kernel_stack(regs, (unsigned long)addr))
+		return addr;
+	else
+		return NULL;
+}
+
+/**
  * regs_get_kernel_stack_nth() - get Nth entry of the stack
  * @regs:	pt_regs which contains kernel stack pointer.
  * @n:		stack entry number.
@@ -248,14 +269,44 @@ static inline int regs_within_kernel_stack(struct pt_regs *regs,
 static inline unsigned long regs_get_kernel_stack_nth(struct pt_regs *regs,
 						      unsigned int n)
 {
-	unsigned long *addr = (unsigned long *)kernel_stack_pointer(regs);
-	addr += n;
-	if (regs_within_kernel_stack(regs, (unsigned long)addr))
+	unsigned long *addr;
+
+	addr = regs_get_kernel_stack_nth_addr(regs, n);
+	if (addr)
 		return *addr;
 	else
 		return 0;
 }
 
+/* To avoid include hell, we can't include uaccess.h */
+extern long probe_kernel_read(void *dst, const void *src, size_t size);
+
+/**
+ * regs_get_kernel_stack_nth_safe() - safely get Nth entry of the stack
+ * @regs:	pt_regs which contains kernel stack pointer.
+ * @n:		stack entry number.
+ *
+ * Same as regs_get_kernel_stack_nth(), but references the stack value
+ * with a probe_kernel_read() in case there's a bad stack pointer, it
+ * will not cause a bad memory access. If the @n is not on the stack,
+ * or a bad memory access happened, it returns zero.
+ */
+static inline unsigned long regs_get_kernel_stack_nth_safe(struct pt_regs *regs,
+							   unsigned int n)
+{
+	unsigned long *addr;
+	unsigned long val;
+	long ret;
+
+	addr = regs_get_kernel_stack_nth_addr(regs, n);
+	if (addr) {
+		ret = probe_kernel_read(&val, addr, sizeof(val));
+		if (!ret)
+			return val;
+	}
+	return 0;
+}
+
 /**
  * regs_get_kernel_argument() - get Nth function argument in kernel
  * @regs:	pt_regs of that context
-- 
2.13.6