77cf2b6dee
Currently we over-estimate the region of stack which must be erased. To determine the region to be erased, we scan downwards for a contiguous block of poison values (or the low bound of the stack). There are a few minor problems with this today: * When we find a block of poison values, we include this block within the region to erase. As this is included within the region to erase, this causes us to redundantly overwrite 'STACKLEAK_SEARCH_DEPTH' (128) bytes with poison. * As the loop condition checks 'poison_count <= depth', it will run an additional iteration after finding the contiguous block of poison, decrementing 'erase_low' once more than necessary. As this is included within the region to erase, this causes us to redundantly overwrite an additional unsigned long with poison. * As we always decrement 'erase_low' after checking an element on the stack, we always include the element below this within the region to erase. As this is included within the region to erase, this causes us to redundantly overwrite an additional unsigned long with poison. Note that this is not a functional problem. As the loop condition checks 'erase_low > task_stack_low', we'll never clobber the STACK_END_MAGIC. As we always decrement 'erase_low' after this, we'll never fail to erase the element immediately above the STACK_END_MAGIC. In total, this can cause us to erase `128 + 2 * sizeof(unsigned long)` bytes more than necessary, which is unfortunate. This patch reworks the logic to find the address immediately above the poisoned region, by finding the lowest non-poisoned address. This is factored into a stackleak_find_top_of_poison() helper both for clarity and so that this can be shared with the LKDTM test in subsequent patches. Signed-off-by: Mark Rutland <mark.rutland@arm.com> Cc: Alexander Popov <alex.popov@linux.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andy Lutomirski <luto@kernel.org> Cc: Kees Cook <keescook@chromium.org> Signed-off-by: Kees Cook <keescook@chromium.org> Link: https://lore.kernel.org/r/20220427173128.2603085-8-mark.rutland@arm.com
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.