On Fri, Mar 12, 2021 at 03:24PM +0100, Andrey Konovalov wrote: > 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. > > Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx> Reviewed-by: Marco Elver <elver@xxxxxxxxxx> > --- > Documentation/dev-tools/kasan.rst | 31 ++++++++++++++----------------- > 1 file changed, 14 insertions(+), 17 deletions(-) > > diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst > index 2744ae6347c6..d0c1796122df 100644 > --- a/Documentation/dev-tools/kasan.rst > +++ b/Documentation/dev-tools/kasan.rst > @@ -304,14 +304,11 @@ 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 > ~~~~~~~~~~~~~~~~~ > @@ -323,10 +320,9 @@ page is mapped over the shadow area. This read-only shadow page > 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 > @@ -337,9 +333,10 @@ 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 > @@ -358,10 +355,10 @@ 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 > -- > 2.31.0.rc2.261.g7f71774620-goog >
