On 09/22/23 at 08:25pm, Kees Cook wrote: > On Sat, Sep 23, 2023 at 08:46:47AM +0800, Baoquan He wrote: > > On 09/22/23 at 10:52am, Kees Cook wrote: > > > Prepare for the coming implementation by GCC and Clang of the __counted_by > > > attribute. Flexible array members annotated with __counted_by can have > > > their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS > > > (for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family > > > functions). > > > > > > As found with Coccinelle[1], add __counted_by for struct crash_mem. > > > > > > [1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci > > > > > > Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx> > > > Cc: kexec@xxxxxxxxxxxxxxxxxxx > > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> > > > --- > > > include/linux/crash_core.h | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/include/linux/crash_core.h b/include/linux/crash_core.h > > > index 3426f6eef60b..5126a4fecb44 100644 > > > --- a/include/linux/crash_core.h > > > +++ b/include/linux/crash_core.h > > > @@ -131,7 +131,7 @@ static inline void __init reserve_crashkernel_generic(char *cmdline, > > > struct crash_mem { > > > unsigned int max_nr_ranges; > > > unsigned int nr_ranges; > > > - struct range ranges[]; > > > + struct range ranges[] __counted_by(max_nr_ranges); > > > > This __counted_by() only makes sense when there's a obvious upper > > boundary, max_nr_ranges in this case. > > Yes; it's designed to be the array element count used for the > allocation. For example with the above case: > > nr_ranges += 2; > cmem = vzalloc(struct_size(cmem, ranges, nr_ranges)); > if (!cmem) > return NULL; > > cmem->max_nr_ranges = nr_ranges; > cmem->nr_ranges = 0; > > nr_ranges is the max count of the elements. > > _However_, if a structure (like this one) has _two_ counters, one for > "in use" and another for "max available", __counted_by could specify the > "in use" case, as long as array indexing only happens when that "in use" > has been updated. So, if it were: > > struct crash_mem { > unsigned int max_nr_ranges; > unsigned int nr_ranges; > struct range ranges[] __counted_by(nr_ranges); > }; > > then this would trigger the bounds checking: > > cmem->ranges[0] = some_range; /* "nr_ranges" is still 0 so index 0 isn't allowed */ > cmem->nr_ranges ++; > > but this would not: > > cmem->nr_ranges ++; /* index 0 is now available for use. */ > cmem->ranges[0] = some_range; > > > This heavily depends and isn't much in kernel? > > Which "this" do you mean? The tracking of max allocation is common. > Tracking max and "in use" happens in some places (like here), but is > less common. I thought usually it may not have a max counter of the variable length array embeded in struct, seems I was wrong. Here 'this' means the __counted_by() adding for the variable length array. > > > E.g struct swap_info_struct->avail_lists[]. > > This is even less common: tracking the count externally from the struct, > as done there with nr_node_ids. Shakeel asked a very similar question > and also pointed out nr_node_ids: > https://lore.kernel.org/all/202309221128.6AC35E3@keescook/ > > > Just curious, not related to this patch though. > > I'm happy to answer questions! Yeah, as I said in the above thread, > I expect to expand what __counted_by can use, and I suspect (hope) > a global would be easier to add than an arbitrary expression. :) Thanks a lot for these explanation, Kees. LGTM, Acked-by: Baoquan He <bhe@xxxxxxxxxx> _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
