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[46.135.88.122]) by smtp.gmail.com with ESMTPSA id d24-20020a056402517800b0049e249c0e56sm2539287ede.56.2023.01.25.07.56.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 25 Jan 2023 07:56:04 -0800 (PST) From: Mateusz Guzik To: viro@zeniv.linux.org.uk Cc: serge@hallyn.com, torvalds@linux-foundation.org, paul@paul-moore.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, Mateusz Guzik Subject: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible Date: Wed, 25 Jan 2023 16:55:57 +0100 Message-Id: <20230125155557.37816-2-mjguzik@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20230125155557.37816-1-mjguzik@gmail.com> References: <20230125155557.37816-1-mjguzik@gmail.com> MIME-Version: 1.0 X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FROM, RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= X-GMAIL-THRID: =?utf-8?q?1756012183376663398?= X-GMAIL-MSGID: =?utf-8?q?1756012183376663398?= access(2) remains commonly used, for example on exec: access("/etc/ld.so.preload", R_OK) or when running gcc: strace -c gcc empty.c % time seconds usecs/call calls errors syscall ------ ----------- ----------- --------- --------- ---------------- 0.00 0.000000 0 42 26 access It falls down to do_faccessat without the AT_EACCESS flag, which in turn results in allocation of new creds in order to modify fsuid/fsgid and caps. This is a very expensive process single-threaded and most notably multi-threaded, with numerous structures getting refed and unrefed on imminent new cred destruction. Turns out for typical consumers the resulting creds would be identical and this can be checked upfront, avoiding the hard work. An access benchmark plugged into will-it-scale running on Cascade Lake shows: test proc before after access1 1 1310582 2908735 (+121%) # distinct files access1 24 4716491 63822173 (+1353%) # distinct files access2 24 2378041 5370335 (+125%) # same file The above benchmarks are not integrated into will-it-scale, but can be found in a pull request: https://github.com/antonblanchard/will-it-scale/pull/36/files Signed-off-by: Mateusz Guzik v3: - add a comment warning about changing access_override_creds v2: - fix current->cred usage warn reported by the kernel test robot Link: https://lore.kernel.org/all/202301150709.9EC6UKBT-lkp@intel.com/ --- fs/open.c | 38 +++++++++++++++++++++++++++++++++++++- 1 file changed, 37 insertions(+), 1 deletion(-) diff --git a/fs/open.c b/fs/open.c index 82c1a28b3308..2afed058250c 100644 --- a/fs/open.c +++ b/fs/open.c @@ -367,7 +367,37 @@ COMPAT_SYSCALL_DEFINE6(fallocate, int, fd, int, mode, compat_arg_u64_dual(offset * access() needs to use the real uid/gid, not the effective uid/gid. * We do this by temporarily clearing all FS-related capabilities and * switching the fsuid/fsgid around to the real ones. + * + * Creating new credentials is expensive, so we try to skip doing it, + * which we can if the result would match what we already got. */ +static bool access_need_override_creds(int flags) +{ + const struct cred *cred; + + if (flags & AT_EACCESS) + return false; + + cred = current_cred(); + if (!uid_eq(cred->fsuid, cred->uid) || + !gid_eq(cred->fsgid, cred->gid)) + return true; + + if (!issecure(SECURE_NO_SETUID_FIXUP)) { + kuid_t root_uid = make_kuid(cred->user_ns, 0); + if (!uid_eq(cred->uid, root_uid)) { + if (!cap_isclear(cred->cap_effective)) + return true; + } else { + if (!cap_isidentical(cred->cap_effective, + cred->cap_permitted)) + return true; + } + } + + return false; +} + static const struct cred *access_override_creds(void) { const struct cred *old_cred; @@ -377,6 +407,12 @@ static const struct cred *access_override_creds(void) if (!override_cred) return NULL; + /* + * XXX access_need_override_creds performs checks in hopes of skipping + * this work. Make sure it stays in sync if making any changes in this + * routine. + */ + override_cred->fsuid = override_cred->uid; override_cred->fsgid = override_cred->gid; @@ -436,7 +472,7 @@ static long do_faccessat(int dfd, const char __user *filename, int mode, int fla if (flags & AT_EMPTY_PATH) lookup_flags |= LOOKUP_EMPTY; - if (!(flags & AT_EACCESS)) { + if (access_need_override_creds(flags)) { old_cred = access_override_creds(); if (!old_cred) return -ENOMEM;