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SubjectRe: [PATCH 2/8] pstore: Do not use crash buffer for decompression
On Thu, Nov 01, 2018 at 04:51:54PM -0700, Kees Cook wrote:
> The pre-allocated compression buffer used for crash dumping was also
> being used for decompression. This isn't technically safe, since it's
> possible the kernel may attempt a crashdump while pstore is populating the
> pstore filesystem (and performing decompression). Instead, just allocate

Yeah, that would be bad if it happened ;)

> a separate buffer for decompression. Correctness is preferred over
> performance here.
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> fs/pstore/platform.c | 56 ++++++++++++++++++++------------------------
> 1 file changed, 25 insertions(+), 31 deletions(-)
>
> diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c
> index b821054ca3ed..8b6028948cf3 100644
> --- a/fs/pstore/platform.c
> +++ b/fs/pstore/platform.c
> @@ -258,20 +258,6 @@ static int pstore_compress(const void *in, void *out,
> return outlen;
> }
>
> -static int pstore_decompress(void *in, void *out,
> - unsigned int inlen, unsigned int outlen)
> -{
> - int ret;
> -
> - ret = crypto_comp_decompress(tfm, in, inlen, out, &outlen);
> - if (ret) {
> - pr_err("crypto_comp_decompress failed, ret = %d!\n", ret);
> - return ret;
> - }
> -
> - return outlen;
> -}
> -
> static void allocate_buf_for_compression(void)
> {
> struct crypto_comp *ctx;
> @@ -656,8 +642,9 @@ EXPORT_SYMBOL_GPL(pstore_unregister);
>
> static void decompress_record(struct pstore_record *record)
> {
> + int ret;
> int unzipped_len;

nit: We could get rid of the unzipped_len variable now I think.

> - char *decompressed;
> + char *unzipped, *workspace;
>
> if (!record->compressed)
> return;
> @@ -668,35 +655,42 @@ static void decompress_record(struct pstore_record *record)
> return;
> }
>
> - /* No compression method has created the common buffer. */
> + /* Missing compression buffer means compression was not initialized. */
> if (!big_oops_buf) {
> - pr_warn("no decompression buffer allocated\n");
> + pr_warn("no decompression method initialized!\n");
> return;
> }
>
> - unzipped_len = pstore_decompress(record->buf, big_oops_buf,
> - record->size, big_oops_buf_sz);
> - if (unzipped_len <= 0) {
> - pr_err("decompression failed: %d\n", unzipped_len);
> + /* Allocate enough space to hold max decompression and ECC. */
> + unzipped_len = big_oops_buf_sz;
> + workspace = kmalloc(unzipped_len + record->ecc_notice_size,

Should tihs be unzipped_len + record->ecc_notice_size + 1. The extra byte
being for the NULL character of the ecc notice?

This occurred to me when I saw the + 1 in ram.c. It could be better to just
abstract the size as a macro.

> + GFP_KERNEL);
> + if (!workspace)
> return;
> - }
>
> - /* Build new buffer for decompressed contents. */
> - decompressed = kmalloc(unzipped_len + record->ecc_notice_size,
> - GFP_KERNEL);
> - if (!decompressed) {
> - pr_err("decompression ran out of memory\n");
> + /* After decompression "unzipped_len" is almost certainly smaller. */
> + ret = crypto_comp_decompress(tfm, record->buf, record->size,
> + workspace, &unzipped_len);
> + if (ret) {
> + pr_err("crypto_comp_decompress failed, ret = %d!\n", ret);
> + kfree(workspace);
> return;
> }
> - memcpy(decompressed, big_oops_buf, unzipped_len);
>
> /* Append ECC notice to decompressed buffer. */
> - memcpy(decompressed + unzipped_len, record->buf + record->size,
> + memcpy(workspace + unzipped_len, record->buf + record->size,
> record->ecc_notice_size);
>
> - /* Swap out compresed contents with decompressed contents. */
> + /* Copy decompressed contents into an minimum-sized allocation. */
> + unzipped = kmemdup(workspace, unzipped_len + record->ecc_notice_size,
> + GFP_KERNEL);
> + kfree(workspace);
> + if (!unzipped)
> + return;
> +
> + /* Swap out compressed contents with decompressed contents. */
> kfree(record->buf);
> - record->buf = decompressed;
> + record->buf = unzipped;

Rest of it LGTM, thanks!

- Joel

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