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Subject[PATCH] lib/bch: Remove VLA usage
In the quest to remove all stack VLA usage from the kernel[1], this removes
the on-stack working buffers in favor of pre-allocated working buffers
(which were already used in other places). Since these routines must
already be serialized (since they work on bch->ecc_buf), adding the usage
of bch->ecc_work would be similarly safe. Additionally, since "max m" is
only 15, this was adjusted to just use a fixed size array in those cases.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Signed-off-by: Kees Cook <keescook@chromium.org>
---
This is directed at linux-mtd because it's the only user of this library
and it's how it originally entered the kernel tree...
---
include/linux/bch.h | 4 ++--
lib/bch.c | 27 +++++++++++++++------------
2 files changed, 17 insertions(+), 14 deletions(-)

diff --git a/include/linux/bch.h b/include/linux/bch.h
index 295b4ef153bb..4d46e6a73319 100644
--- a/include/linux/bch.h
+++ b/include/linux/bch.h
@@ -39,7 +39,7 @@
* @a_log_tab: Galois field GF(2^m) log lookup table
* @mod8_tab: remainder generator polynomial lookup tables
* @ecc_buf: ecc parity words buffer
- * @ecc_buf2: ecc parity words buffer
+ * @ecc_work: ecc parity words working buffer
* @xi_tab: GF(2^m) base for solving degree 2 polynomial roots
* @syn: syndrome buffer
* @cache: log-based polynomial representation buffer
@@ -57,7 +57,7 @@ struct bch_control {
uint16_t *a_log_tab;
uint32_t *mod8_tab;
uint32_t *ecc_buf;
- uint32_t *ecc_buf2;
+ uint32_t *ecc_work;
unsigned int *xi_tab;
unsigned int *syn;
int *cache;
diff --git a/lib/bch.c b/lib/bch.c
index bc89dfe4d1b3..f14eac93ecc4 100644
--- a/lib/bch.c
+++ b/lib/bch.c
@@ -78,10 +78,12 @@
#define GF_M(_p) (CONFIG_BCH_CONST_M)
#define GF_T(_p) (CONFIG_BCH_CONST_T)
#define GF_N(_p) ((1 << (CONFIG_BCH_CONST_M))-1)
+#define BCH_MAX_M (CONFIG_BCH_CONST_M)
#else
#define GF_M(_p) ((_p)->m)
#define GF_T(_p) ((_p)->t)
#define GF_N(_p) ((_p)->n)
+#define BCH_MAX_M 15
#endif

#define BCH_ECC_WORDS(_p) DIV_ROUND_UP(GF_M(_p)*GF_T(_p), 32)
@@ -187,7 +189,7 @@ void encode_bch(struct bch_control *bch, const uint8_t *data,
const unsigned int l = BCH_ECC_WORDS(bch)-1;
unsigned int i, mlen;
unsigned long m;
- uint32_t w, r[l+1];
+ uint32_t w;
const uint32_t * const tab0 = bch->mod8_tab;
const uint32_t * const tab1 = tab0 + 256*(l+1);
const uint32_t * const tab2 = tab1 + 256*(l+1);
@@ -198,7 +200,7 @@ void encode_bch(struct bch_control *bch, const uint8_t *data,
/* load ecc parity bytes into internal 32-bit buffer */
load_ecc8(bch, bch->ecc_buf, ecc);
} else {
- memset(bch->ecc_buf, 0, sizeof(r));
+ memset(bch->ecc_work, 0, bch->ecc_bytes);
}

/* process first unaligned data bytes */
@@ -215,7 +217,7 @@ void encode_bch(struct bch_control *bch, const uint8_t *data,
mlen = len/4;
data += 4*mlen;
len -= 4*mlen;
- memcpy(r, bch->ecc_buf, sizeof(r));
+ memcpy(bch->ecc_work, bch->ecc_buf, bch->ecc_bytes);

/*
* split each 32-bit word into 4 polynomials of weight 8 as follows:
@@ -229,6 +231,8 @@ void encode_bch(struct bch_control *bch, const uint8_t *data,
* xxxxxxxx yyyyyyyy zzzzzzzz tttttttt mod g = r0^r1^r2^r3
*/
while (mlen--) {
+ uint32_t *r = bch->ecc_work;
+
/* input data is read in big-endian format */
w = r[0]^cpu_to_be32(*pdata++);
p0 = tab0 + (l+1)*((w >> 0) & 0xff);
@@ -241,7 +245,7 @@ void encode_bch(struct bch_control *bch, const uint8_t *data,

r[l] = p0[l]^p1[l]^p2[l]^p3[l];
}
- memcpy(bch->ecc_buf, r, sizeof(r));
+ memcpy(bch->ecc_buf, bch->ecc_work, bch->ecc_bytes);

/* process last unaligned bytes */
if (len)
@@ -434,7 +438,7 @@ static int solve_linear_system(struct bch_control *bch, unsigned int *rows,
{
const int m = GF_M(bch);
unsigned int tmp, mask;
- int rem, c, r, p, k, param[m];
+ int rem, c, r, p, k, param[BCH_MAX_M];

k = 0;
mask = 1 << m;
@@ -1009,10 +1013,10 @@ int decode_bch(struct bch_control *bch, const uint8_t *data, unsigned int len,
}
/* load received ecc or assume it was XORed in calc_ecc */
if (recv_ecc) {
- load_ecc8(bch, bch->ecc_buf2, recv_ecc);
+ load_ecc8(bch, bch->ecc_work, recv_ecc);
/* XOR received and calculated ecc */
for (i = 0, sum = 0; i < (int)ecc_words; i++) {
- bch->ecc_buf[i] ^= bch->ecc_buf2[i];
+ bch->ecc_buf[i] ^= bch->ecc_work[i];
sum |= bch->ecc_buf[i];
}
if (!sum)
@@ -1114,7 +1118,7 @@ static int build_deg2_base(struct bch_control *bch)
{
const int m = GF_M(bch);
int i, j, r;
- unsigned int sum, x, y, remaining, ak = 0, xi[m];
+ unsigned int sum, x, y, remaining, ak = 0, xi[BCH_MAX_M];

/* find k s.t. Tr(a^k) = 1 and 0 <= k < m */
for (i = 0; i < m; i++) {
@@ -1254,7 +1258,6 @@ struct bch_control *init_bch(int m, int t, unsigned int prim_poly)
struct bch_control *bch = NULL;

const int min_m = 5;
- const int max_m = 15;

/* default primitive polynomials */
static const unsigned int prim_poly_tab[] = {
@@ -1270,7 +1273,7 @@ struct bch_control *init_bch(int m, int t, unsigned int prim_poly)
goto fail;
}
#endif
- if ((m < min_m) || (m > max_m))
+ if ((m < min_m) || (m > BCH_MAX_M))
/*
* values of m greater than 15 are not currently supported;
* supporting m > 15 would require changing table base type
@@ -1300,7 +1303,7 @@ struct bch_control *init_bch(int m, int t, unsigned int prim_poly)
bch->a_log_tab = bch_alloc((1+bch->n)*sizeof(*bch->a_log_tab), &err);
bch->mod8_tab = bch_alloc(words*1024*sizeof(*bch->mod8_tab), &err);
bch->ecc_buf = bch_alloc(words*sizeof(*bch->ecc_buf), &err);
- bch->ecc_buf2 = bch_alloc(words*sizeof(*bch->ecc_buf2), &err);
+ bch->ecc_work = bch_alloc(words*sizeof(*bch->ecc_work), &err);
bch->xi_tab = bch_alloc(m*sizeof(*bch->xi_tab), &err);
bch->syn = bch_alloc(2*t*sizeof(*bch->syn), &err);
bch->cache = bch_alloc(2*t*sizeof(*bch->cache), &err);
@@ -1349,7 +1352,7 @@ void free_bch(struct bch_control *bch)
kfree(bch->a_log_tab);
kfree(bch->mod8_tab);
kfree(bch->ecc_buf);
- kfree(bch->ecc_buf2);
+ kfree(bch->ecc_work);
kfree(bch->xi_tab);
kfree(bch->syn);
kfree(bch->cache);
--
2.17.0

--
Kees Cook
Pixel Security

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