From: Andrey Konovalov <andreyknvl@xxxxxxxxxx> Subject: kasan: docs: update shadow memory section Update the "Shadow memory" section in KASAN documentation: - Rearrange the introduction paragraph do it doesn't give a "KASAN has an issue" impression. - Update the list of architectures with vmalloc support. - Punctuation, readability, and other minor clean-ups. Link: https://lkml.kernel.org/r/00f8c38b0fd5290a3f4dced04eaba41383e67e14.1615559068.git.andreyknvl@xxxxxxxxxx Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx> Reviewed-by: Marco Elver <elver@xxxxxxxxxx> Cc: Alexander Potapenko <glider@xxxxxxxxxx> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- Documentation/dev-tools/kasan.rst | 31 ++++++++++++---------------- 1 file changed, 14 insertions(+), 17 deletions(-) --- a/Documentation/dev-tools/kasan.rst~kasan-docs-update-shadow-memory-section +++ a/Documentation/dev-tools/kasan.rst @@ -313,14 +313,11 @@ checking gets disabled. Shadow memory ------------- -The kernel maps memory in a number of different parts of the address -space. This poses something of a problem for KASAN, which requires -that all addresses accessed by instrumented code have a valid shadow -region. - -The range of kernel virtual addresses is large: there is not enough -real memory to support a real shadow region for every address that -could be accessed by the kernel. +The kernel maps memory in several different parts of the address space. +The range of kernel virtual addresses is large: there is not enough real +memory to support a real shadow region for every address that could be +accessed by the kernel. Therefore, KASAN only maps real shadow for certain +parts of the address space. Default behaviour ~~~~~~~~~~~~~~~~~ @@ -332,10 +329,9 @@ page is mapped over the shadow area. Thi declares all memory accesses as permitted. This presents a problem for modules: they do not live in the linear -mapping, but in a dedicated module space. By hooking in to the module -allocator, KASAN can temporarily map real shadow memory to cover -them. This allows detection of invalid accesses to module globals, for -example. +mapping but in a dedicated module space. By hooking into the module +allocator, KASAN temporarily maps real shadow memory to cover them. +This allows detection of invalid accesses to module globals, for example. This also creates an incompatibility with ``VMAP_STACK``: if the stack lives in vmalloc space, it will be shadowed by the read-only page, and @@ -346,9 +342,10 @@ CONFIG_KASAN_VMALLOC ~~~~~~~~~~~~~~~~~~~~ With ``CONFIG_KASAN_VMALLOC``, KASAN can cover vmalloc space at the -cost of greater memory usage. Currently this is only supported on x86. +cost of greater memory usage. Currently, this is supported on x86, +riscv, s390, and powerpc. -This works by hooking into vmalloc and vmap, and dynamically +This works by hooking into vmalloc and vmap and dynamically allocating real shadow memory to back the mappings. Most mappings in vmalloc space are small, requiring less than a full @@ -367,10 +364,10 @@ memory. To avoid the difficulties around swapping mappings around, KASAN expects that the part of the shadow region that covers the vmalloc space will -not be covered by the early shadow page, but will be left -unmapped. This will require changes in arch-specific code. +not be covered by the early shadow page but will be left unmapped. +This will require changes in arch-specific code. -This allows ``VMAP_STACK`` support on x86, and can simplify support of +This allows ``VMAP_STACK`` support on x86 and can simplify support of architectures that do not have a fixed module region. For developers _
