On Fri, Jun 11, 2021 at 12:58:20PM +0100, Andre Przywara wrote:
> fdt_num_mem_rsv() can return a negative error value, when the memreserve
> block is not properly terminated or is exceeding its advertised size,
> This can happen on malformed DTBs, and FDT_RO_PROBE() does not cover this.
>
> Check the return value of fdt_num_mem_rsv() first for an error
> condition, and also check if the total size of the memreserve block
> would provoke a signed integer overflow.
>
> This fixes issues with malformed DT blobs, where we would end up with
> negative offsets into buffers.
>
> Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
> ---
> libfdt/fdt_rw.c | 19 +++++++++++++++----
> 1 file changed, 15 insertions(+), 4 deletions(-)
>
> diff --git a/libfdt/fdt_rw.c b/libfdt/fdt_rw.c
> index 3621d36..a0f03d0 100644
> --- a/libfdt/fdt_rw.c
> +++ b/libfdt/fdt_rw.c
> @@ -427,8 +427,13 @@ int fdt_open_into(const void *fdt, void *buf, int bufsize)
>
> FDT_RO_PROBE(fdt);
>
> - mem_rsv_size = (fdt_num_mem_rsv(fdt)+1)
> - * sizeof(struct fdt_reserve_entry);
> + mem_rsv_size = fdt_num_mem_rsv(fdt);
> + if (mem_rsv_size < 0)
You want a can_assume(VALID_DTB) on this test as well. The people
doing fdt on miniscule first stage roms really hate every extra
instruction.
> + return mem_rsv_size;
> +
> + mem_rsv_size = (mem_rsv_size + 1) * sizeof(struct fdt_reserve_entry);
> + if (!can_assume(VALID_DTB) && mem_rsv_size < 0)
> + return -FDT_ERR_NOSPACE;
This second test is not necessary. fdt_num_mem_rsv() implicitly
checks for a minimally sane mem reserve block, which implies its size
lies within totalsize, which in turn is <= INT_MAX.
If it were necessary, I don't think it's quite safe, because signed
overflow is UB, IIUC.
> if (can_assume(LATEST) || fdt_version(fdt) >= 17) {
> struct_size = fdt_size_dt_struct(fdt);
> @@ -490,8 +495,14 @@ int fdt_pack(void *fdt)
>
> FDT_RW_PROBE(fdt);
>
> - mem_rsv_size = (fdt_num_mem_rsv(fdt)+1)
> - * sizeof(struct fdt_reserve_entry);
> + mem_rsv_size = fdt_num_mem_rsv(fdt);
> + if (mem_rsv_size < 0)
> + return mem_rsv_size;
> +
> + mem_rsv_size = (mem_rsv_size + 1) * sizeof(struct fdt_reserve_entry);
> + if (!can_assume(VALID_DTB) && mem_rsv_size < 0)
> + return -FDT_ERR_NOSPACE;
Same comments here.
> fdt_packblocks_(fdt, fdt, mem_rsv_size, fdt_size_dt_struct(fdt),
> fdt_size_dt_strings(fdt));
> fdt_set_totalsize(fdt, fdt_data_size_(fdt));
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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