Avoid overlapping memcpy() with invalid input with in-place decompression.
With valid files, the safety margin described in lib/decompress_unxz.c
ensures that these buffers cannot overlap. But if the uncompressed size
of the input is larger than the caller thought, which is possible when
the input file is invalid/corrupt, the buffers can overlap. Obviously
the result will then be garbage (and usually the decoder will return
an error too) but no other harm will happen when such an over-run occurs.
This change only affects uncompressed LZMA2 chunks and so this
should have no effect on performance.
diff --git a/linux/lib/decompress_unxz.c b/linux/lib/decompress_unxz.c
index 9f34eb5..3033b20 100644
--- a/linux/lib/decompress_unxz.c
+++ b/linux/lib/decompress_unxz.c
@@ -167,7 +167,7 @@
* memeq and memzero are not used much and any remotely sane implementation
* is fast enough. memcpy/memmove speed matters in multi-call mode, but
* the kernel image is decompressed in single-call mode, in which only
- * memcpy speed can matter and only if there is a lot of uncompressible data
+ * memmove speed can matter and only if there is a lot of uncompressible data
* (LZMA2 stores uncompressible chunks in uncompressed form). Thus, the
* functions below should just be kept small; it's probably not worth
* optimizing for speed.
diff --git a/linux/lib/xz/xz_dec_lzma2.c b/linux/lib/xz/xz_dec_lzma2.c
index 156f26f..dd80989 100644
--- a/linux/lib/xz/xz_dec_lzma2.c
+++ b/linux/lib/xz/xz_dec_lzma2.c
@@ -387,7 +387,14 @@
*left -= copy_size;
- memcpy(dict->buf + dict->pos, b->in + b->in_pos, copy_size);
+ /*
+ * If doing in-place decompression in single-call mode and the
+ * uncompressed size of the file is larger than the caller
+ * thought (i.e. it is invalid input!), the buffers below may
+ * overlap and cause undefined behavior with memcpy().
+ * With valid inputs memcpy() would be fine here.
+ */
+ memmove(dict->buf + dict->pos, b->in + b->in_pos, copy_size);
dict->pos += copy_size;
if (dict->full < dict->pos)
@@ -397,7 +404,11 @@
if (dict->pos == dict->end)
dict->pos = 0;
- memcpy(b->out + b->out_pos, b->in + b->in_pos,
+ /*
+ * Like above but for multi-call mode: use memmove()
+ * to avoid undefined behavior with invalid input.
+ */
+ memmove(b->out + b->out_pos, b->in + b->in_pos,
copy_size);
}
@@ -421,6 +432,12 @@
if (dict->pos == dict->end)
dict->pos = 0;
+ /*
+ * These buffers cannot overlap even if doing in-place
+ * decompression because in multi-call mode dict->buf
+ * has been allocated by us in this file; it's not
+ * provided by the caller like in single-call mode.
+ */
memcpy(b->out + b->out_pos, dict->buf + dict->start,
copy_size);
}