On Mon, Sep 11, 2023 at 1:44 PM Marco Elver <elver@xxxxxxxxxx> wrote: > > On Thu, 7 Sept 2023 at 15:06, Alexander Potapenko <glider@xxxxxxxxxx> wrote: > > > > kmsan_internal_memmove_metadata() is the function that implements > > copying metadata every time memcpy()/memmove() is called. > > Because shadow memory stores 1 byte per each byte of kernel memory, > > copying the shadow is trivial and can be done by a single memmove() > > call. > > Origins, on the other hand, are stored as 4-byte values corresponding > > to every aligned 4 bytes of kernel memory. Therefore, if either the > > source or the destination of kmsan_internal_memmove_metadata() is > > unaligned, the number of origin slots corresponding to the source or > > destination may differ: > > > > 1) memcpy(0xffff888080a00000, 0xffff888080900000, 4) > > copies 1 origin slot into 1 origin slot: > > > > src (0xffff888080900000): xxxx > > src origins: o111 > > dst (0xffff888080a00000): xxxx > > dst origins: o111 > > > > 2) memcpy(0xffff888080a00001, 0xffff888080900000, 4) > > copies 1 origin slot into 2 origin slots: > > > > src (0xffff888080900000): xxxx > > src origins: o111 > > dst (0xffff888080a00000): .xxx x... > > dst origins: o111 o111 > > > > 3) memcpy(0xffff888080a00000, 0xffff888080900001, 4) > > copies 2 origin slots into 1 origin slot: > > > > src (0xffff888080900000): .xxx x... > > src origins: o111 o222 > > dst (0xffff888080a00000): xxxx > > dst origins: o111 > > (or o222) > > > > Previously, kmsan_internal_memmove_metadata() tried to solve this > > problem by copying min(src_slots, dst_slots) as is and cloning the > > missing slot on one of the ends, if needed. > > This was error-prone even in the simple cases where 4 bytes were copied, > > and did not account for situations where the total number of nonzero > > origin slots could have increased by more than one after copying: > > > > memcpy(0xffff888080a00000, 0xffff888080900002, 8) > > > > src (0xffff888080900002): ..xx .... xx.. > > src origins: o111 0000 o222 > > dst (0xffff888080a00000): xx.. ..xx > > o111 0000 > > (or 0000 o222) > > > > The new implementation simply copies the shadow byte by byte, and > > updates the corresponding origin slot, if the shadow byte is nonzero. > > This approach can handle complex cases with mixed initialized and > > uninitialized bytes. Similarly to KMSAN inline instrumentation, latter > > writes to bytes sharing the same origin slots take precedence. > > > > Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx> > > I think this needs a Fixes tag. Thanks, will add in v2! > Also, is this corner case exercised by one of the KMSAN KUnit test cases? Ditto > Otherwise, > > Acked-by: Marco Elver <elver@xxxxxxxxxx>
