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Mark Rutland
e85094c31d
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 implementation. The arm64 implementation of current_top_of_stack() was added specifically for stackleak in commit:0b3e336601("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 wildly: * 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 stack. 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:0b3e336601("arm64: Add support for STACKLEAK gcc plugin") Signed-off-by: Mark Rutland <[email protected]> Cc: Alexander Popov <[email protected]> Cc: Andrew Morton <[email protected]> Cc: Andy Lutomirski <[email protected]> Cc: Catalin Marinas <[email protected]> Cc: Kees Cook <[email protected]> Cc: Will Deacon <[email protected]> Acked-by: Catalin Marinas <[email protected]> Signed-off-by: Kees Cook <[email protected]> Link: https://lore.kernel.org/r/[email protected]
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.