[PATCH 1/8] arm64: stackleak: fix current_top_of_stack()

From: Mark Rutland
Date: Mon Apr 25 2022 - 07:56:30 EST

Due to some historical confusion, arm64's current_top_of_stack() isn't
what the stackleak code expects. This could in theory result in a number
of problems, and practically results in an unnecessary performance hit.
We can avoid this by aligning the arm64 implementation with the x86

The arm64 implementation of current_top_of_stack() was added
specifically for stackleak in commit:

0b3e336601b82c6a ("arm64: Add support for STACKLEAK gcc plugin")

This was intended to be equivalent to the x86 implementation, but the
implementation, semantics, and performance characteristics differ

* On x86, current_top_of_stack() returns the top of the current task's
task stack, regardless of which stack is in active use.

The implementation accesses a percpu variable which the x86 entry code
maintains, and returns the location immediately above the pt_regs on
the task stack (above which x86 has some padding).

* On arm64 current_top_of_stack() returns the top of the stack in active
use (i.e. the one which is currently being used).

The implementation checks the SP against a number of
potentially-accessible stacks, and will BUG() if no stack is found.

The core stackleak_erase() code determines the upper bound of stack to
erase with:

| if (on_thread_stack())
| boundary = current_stack_pointer;
| else
| boundary = current_top_of_stack();

On arm64 stackleak_erase() is always called on a task stack, and
on_thread_stack() should always be true. On x86, stackleak_erase() is
mostly called on a trampoline stack, and is sometimes called on a task

Currently, this results in a lot of unnecessary code being generated for
arm64 for the impossible !on_thread_stack() case. Some of this is
inlined, bloating stackleak_erase(), while portions of this are left
out-of-line and permitted to be instrumented (which would be a
functional problem if that code were reachable).

As a first step towards improving this, this patch aligns arm64's
implementation of current_top_of_stack() with x86's, always returning
the top of the current task's stack. With GCC 11.1.0 this results in the
bulk of the unnecessary code being removed, including all of the
out-of-line instrumentable code.

While I don't believe there's a functional problem in practice I've
marked this as a fix since the semantic was clearly wrong, the fix
itself is simple, and other code might rely upon this in future.

Fixes: 0b3e336601b82c6a ("arm64: Add support for STACKLEAK gcc plugin")
Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Alexander Popov <alex.popov@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
arch/arm64/include/asm/processor.h | 10 ++++------
1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 73e38d9a540ce..6b1a12c23fe77 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -381,12 +381,10 @@ long get_tagged_addr_ctrl(struct task_struct *task);
* of header definitions for the use of task_stack_page.

-#define current_top_of_stack() \
-({ \
- struct stack_info _info; \
- BUG_ON(!on_accessible_stack(current, current_stack_pointer, 1, &_info)); \
- _info.high; \
+ * The top of the current task's task stack
+ */
+#define current_top_of_stack() ((unsigned long)current->stack + THREAD_SIZE)
#define on_thread_stack() (on_task_stack(current, current_stack_pointer, 1, NULL))

#endif /* __ASSEMBLY__ */