[PATCH] crypto: scomp - Fix off-by-one bug when calculating last page

Herbert Xu
2025-04-21T03:31:58Z

On Sun, Apr 20, 2025 at 04:35:44PM -0700, Nhat Pham wrote:
>
> Anyhow, this looks like a crypto/compression infra bug. Herbert, does
> this ring any bell for you?

Yes this looks like an off-by-one bug in the new scomp scratch
code.

---8<---
Fix off-by-one bug in the last page calculation for src and dst.

Reported-by: Nhat Pham <nphamcs@gmail.com>
Fixes: 2d3553ecb4e3 ("crypto: scomp - Remove support for some non-trivial SG lists")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/crypto/scompress.c b/crypto/scompress.c
index 5762fcc63b51..36934c78d127 100644
--- a/crypto/scompress.c
+++ b/crypto/scompress.c
@@ -215,8 +215,8 @@ static int scomp_acomp_comp_decomp(struct acomp_req *req, int dir)
spage = nth_page(spage, soff / PAGE_SIZE);
soff = offset_in_page(soff);

- n = slen / PAGE_SIZE;
- n += (offset_in_page(slen) + soff - 1) / PAGE_SIZE;
+ n = (slen - 1) / PAGE_SIZE;
+ n += (offset_in_page(slen - 1) + soff) / PAGE_SIZE;
if (PageHighMem(nth_page(spage, n)) &&
size_add(soff, slen) > PAGE_SIZE)
break;
@@ -243,9 +243,9 @@ static int scomp_acomp_comp_decomp(struct acomp_req *req, int dir)
dpage = nth_page(dpage, doff / PAGE_SIZE);
doff = offset_in_page(doff);

- n = dlen / PAGE_SIZE;
- n += (offset_in_page(dlen) + doff - 1) / PAGE_SIZE;
- if (PageHighMem(dpage + n) &&
+ n = (dlen - 1) / PAGE_SIZE;
+ n += (offset_in_page(dlen - 1) + doff) / PAGE_SIZE;
+ if (PageHighMem(nth_page(dpage, n)) &&
size_add(doff, dlen) > PAGE_SIZE)
break;
dst = kmap_local_page(dpage) + doff;
--
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt