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Message-ID: <202309251103.4FBC42C4@keescook> Date: Mon, 25 Sep 2023 11:03:34 -0700 From: Kees Cook <keescook@...omium.org> To: Justin Stitt <justinstitt@...gle.com> Cc: Alasdair Kergon <agk@...hat.com>, Mike Snitzer <snitzer@...nel.org>, dm-devel@...hat.com, linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org Subject: Re: [PATCH] dm cache metadata: replace deprecated strncpy with strscpy On Mon, Sep 25, 2023 at 06:13:12AM +0000, Justin Stitt wrote: > `strncpy` is deprecated for use on NUL-terminated destination strings > [1] and as such we should prefer more robust and less ambiguous string > interfaces. > > It seems `cmd->policy_name` is intended to be NUL-terminated based on a > now changed line of code from Commit (c6b4fcbad044e6ff "dm: add cache > target"): > | if (strcmp(cmd->policy_name, policy_name)) { // ... > > However, now a length-bounded strncmp is used: > | if (strncmp(cmd->policy_name, policy_name, sizeof(cmd->policy_name))) > ... which means NUL-terminated may not strictly be required. However, I > believe the intent of the code is clear and we should maintain > NUL-termination of policy_names. > > Moreover, __begin_transaction_flags() zero-allocates `cmd` before > calling read_superblock_fields(): > | cmd = kzalloc(sizeof(*cmd), GFP_KERNEL); > > Also, `disk_super->policy_name` is zero-initialized > | memset(disk_super->policy_name, 0, sizeof(disk_super->policy_name)); > ... therefore any NUL-padding is redundant. > > Considering the above, a suitable replacement is `strscpy` [2] due to > the fact that it guarantees NUL-termination on the destination buffer > without unnecessarily NUL-padding. > > Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1] > Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2] > Link: https://github.com/KSPP/linux/issues/90 > Cc: linux-hardening@...r.kernel.org > Signed-off-by: Justin Stitt <justinstitt@...gle.com> Agreed about the %NUL termination and padding assessment. Reviewed-by: Kees Cook <keescook@...omium.org> -Kees -- Kees Cook
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