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Subject[PATCH v3 2/3] dh key: get rid of stack allocated array
Date
We're interested in getting rid of all of the stack allocated arrays in the
kernel: https://lkml.org/lkml/2018/3/7/621

This particular vla is used as a temporary output buffer in case there is
too much hash output for the destination buffer. Instead, let's just
allocate a buffer that's big enough initially, but only copy back to
userspace the amount that was originally asked for.

v2: allocate enough in the original output buffer vs creating a temporary
output buffer

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
CC: David Howells <dhowells@redhat.com>
CC: James Morris <jmorris@namei.org>
CC: "Serge E. Hallyn" <serge@hallyn.com>
CC: Eric Biggers <ebiggers3@gmail.com>
---
security/keys/dh.c | 29 ++++++++++-------------------
1 file changed, 10 insertions(+), 19 deletions(-)

diff --git a/security/keys/dh.c b/security/keys/dh.c
index d1ea9f325f94..9fecaea6c298 100644
--- a/security/keys/dh.c
+++ b/security/keys/dh.c
@@ -183,24 +183,13 @@ static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
goto err;
}

- if (dlen < h) {
- u8 tmpbuffer[h];
-
- err = crypto_shash_final(desc, tmpbuffer);
- if (err)
- goto err;
- memcpy(dst, tmpbuffer, dlen);
- memzero_explicit(tmpbuffer, h);
- return 0;
- } else {
- err = crypto_shash_final(desc, dst);
- if (err)
- goto err;
+ err = crypto_shash_final(desc, dst);
+ if (err)
+ goto err;

- dlen -= h;
- dst += h;
- counter = cpu_to_be32(be32_to_cpu(counter) + 1);
- }
+ dlen -= h;
+ dst += h;
+ counter = cpu_to_be32(be32_to_cpu(counter) + 1);
}

return 0;
@@ -216,14 +205,16 @@ static int keyctl_dh_compute_kdf(struct kdf_sdesc *sdesc,
{
uint8_t *outbuf = NULL;
int ret;
+ size_t outbuf_len = round_up(buflen,
+ crypto_shash_digestsize(sdesc->shash.tfm));

- outbuf = kmalloc(buflen, GFP_KERNEL);
+ outbuf = kmalloc(outbuf_len, GFP_KERNEL);
if (!outbuf) {
ret = -ENOMEM;
goto err;
}

- ret = kdf_ctr(sdesc, kbuf, kbuflen, outbuf, buflen, lzero);
+ ret = kdf_ctr(sdesc, kbuf, kbuflen, outbuf, outbuf_len, lzero);
if (ret)
goto err;

--
2.17.0
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