On Fri, Aug 16, 2019 at 10:08 AM Mark Rutland <mark.rutland@xxxxxxx> wrote: > > Hi Christophe, > > On Fri, Aug 16, 2019 at 09:47:00AM +0200, Christophe Leroy wrote: > > Le 15/08/2019 à 02:16, Daniel Axtens a écrit : > > > Hook into vmalloc and vmap, and dynamically allocate real shadow > > > memory to back the mappings. > > > > > > Most mappings in vmalloc space are small, requiring less than a full > > > page of shadow space. Allocating a full shadow page per mapping would > > > therefore be wasteful. Furthermore, to ensure that different mappings > > > use different shadow pages, mappings would have to be aligned to > > > KASAN_SHADOW_SCALE_SIZE * PAGE_SIZE. > > > > > > Instead, share backing space across multiple mappings. Allocate > > > a backing page the first time a mapping in vmalloc space uses a > > > particular page of the shadow region. Keep this page around > > > regardless of whether the mapping is later freed - in the mean time > > > the page could have become shared by another vmalloc mapping. > > > > > > This can in theory lead to unbounded memory growth, but the vmalloc > > > allocator is pretty good at reusing addresses, so the practical memory > > > usage grows at first but then stays fairly stable. > > > > I guess people having gigabytes of memory don't mind, but I'm concerned > > about tiny targets with very little amount of memory. I have boards with as > > little as 32Mbytes of RAM. The shadow region for the linear space already > > takes one eighth of the RAM. I'd rather avoid keeping unused shadow pages > > busy. > > I think this depends on how much shadow would be in constant use vs what > would get left unused. If the amount in constant use is sufficiently > large (or the residue is sufficiently small), then it may not be > worthwhile to support KASAN_VMALLOC on such small systems. > > > Each page of shadow memory represent 8 pages of real memory. Could we use > > page_ref to count how many pieces of a shadow page are used so that we can > > free it when the ref count decreases to 0. > > > > > This requires architecture support to actually use: arches must stop > > > mapping the read-only zero page over portion of the shadow region that > > > covers the vmalloc space and instead leave it unmapped. > > > > Why 'must' ? Couldn't we switch back and forth from the zero page to real > > page on demand ? > > > > If the zero page is not mapped for unused vmalloc space, bad memory accesses > > will Oops on the shadow memory access instead of Oopsing on the real bad > > access, making it more difficult to locate and identify the issue. > > I agree this isn't nice, though FWIW this can already happen today for > bad addresses that fall outside of the usual kernel address space. We > could make the !KASAN_INLINE checks resilient to this by using > probe_kernel_read() to check the shadow, and treating unmapped shadow as > poison. Could we instead modify the page fault handlers to detect this case and print a useful message?
