Re: [musl] realloc buffer overflow issue

[Rich Felker <dalias@libc.org>]

On Sun, Feb 28, 2021 at 11:54:58PM +0800, Benedict wrote:
> I found an memory leak issue in my multi-thread, which had fixed issue by the following patch
> 
> 
> 
> https://git.musl-libc.org/cgit/musl/commit/src/malloc?id=3e16313f8fe2ed143ae0267fd79d63014c24779f
> 
> 
> 
> 
> 
> 
> about this patch, I think it involved a bug for realloc function:
> when the request size 'n' (the second paramter passed to realloc)
> lesser than the original memory size 'n0' (the memory block that
> first paramter of realloc pointed to) and the next chunk of 'self '
> is in used state, the realloc should call trim(self, n) to split
> current chunk rather than call malloc, that will cause memcpy buffer
> overflow due to 'n < n0', and it will cause next malloc crash...
> 
> 
> after fix, I think it should be like:
> lock(mal.split_merge_lock);
> 
> 
> size_t nsize = next->csize & C_INUSE ? 0 : CHUNK_SIZE(next);
> if (nsize)) {
> int i = bin_index(nsize);
> lock_bin(i);
> if (!(next->csize & C_INUSE)) {
> unbin(next, i);
> next = NEXT_CHUNK(next);
> self->csize = next->psize = n0+nsize | C_INUSE;
> }
> unlock_bin(i);
> }
>     
>     if(CHUNK_SIZE(self) >= n) {
>         trim(self, n);
>         unlock(mal.split_merge_lock);
> return CHUNK_TO_MEM(self);
>     }
> unlock(mal.split_merge_lock);
> 
> 
> copy_realloc:
> /* As a last resort, allocate a new chunk and copy to it. */
> new = malloc(n-OVERHEAD);
> if (!new) return 0;
> copy_free_ret:
> memcpy(new, p, n0-OVERHEAD);
> free(CHUNK_TO_MEM(self));
> return new;

I haven't looked in detail yet but isn't this the same bug fixed a
week after it was introduced with commits
cb5babdc8d624a3e3e7bea0b4e28a677a2f2fc46 and
fca7428c096066482d8c3f52450810288e27515c ?

Rich