3cbc6c627f
Many libc functions have an option to not follow symbolic links. This is useful to avoid security sensitive code from inadvertantly following attacker supplied symlinks and taking inappropriate action on files it shouldn't. For example, open() has O_NOFOLLOW, chown() has lchown(), stat() has lstat(), etc. There is no such equivalent function for chmod(), such as lchmod(). To address this, POSIX introduced fchmodat(AT_SYMLINK_NOFOLLOW), which is intended to provide a way to perform a chmod operation which doesn't follow symlinks. Currently, the Linux kernel doesn't implement AT_SYMLINK_NOFOLLOW. In GLIBC, attempting to use the AT_SYMLINK_NOFOLLOW flag causes fchmodat to return ENOTSUP. Details are in "man fchmodat". Bionic currently differs from GLIBC in that AT_SYMLINK_NOFOLLOW is silently ignored and treated as if the flag wasn't present. This patch provides a userspace implementation of AT_SYMLINK_NOFOLLOW for bionic. Using open(O_PATH | O_NOFOLLOW), we can provide a way to atomically change the permissions on files without worrying about race conditions. As part of this change, we add support for fchmod on O_PATH file descriptors, because it's relatively straight forward and could be useful in the future. The basic idea behind this implementation comes from https://sourceware.org/bugzilla/show_bug.cgi?id=14578 , specifically comment #10. Change-Id: I1eba0cdb2c509d9193ceecf28f13118188a3cfa7
72 lines
2.6 KiB
C++
72 lines
2.6 KiB
C++
/*
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* Copyright (C) 2015 The Android Open Source Project
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* * Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* * Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in
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* the documentation and/or other materials provided with the
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* distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
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* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
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* COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
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* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
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* BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
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* OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
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* AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
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* OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*/
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#include <fcntl.h>
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#include <sys/stat.h>
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#include <sys/types.h>
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#include <errno.h>
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#include <unistd.h>
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#include <stdio.h>
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extern "C" int __fchmod(int, mode_t);
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int fchmod(int fd, mode_t mode) {
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int saved_errno = errno;
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int result = __fchmod(fd, mode);
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if ((result == 0) || (errno != EBADF)) {
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return result;
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}
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// fd could be an O_PATH file descriptor, and the kernel
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// may not directly support fchmod() on such a file descriptor.
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// Use /proc/self/fd instead to emulate this support.
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// https://sourceware.org/bugzilla/show_bug.cgi?id=14578
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//
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// As of February 2015, there are no kernels which support fchmod
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// on an O_PATH file descriptor, and "man open" documents fchmod
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// on O_PATH file descriptors as returning EBADF.
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int fd_flag = fcntl(fd, F_GETFL);
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if ((fd_flag == -1) || ((fd_flag & O_PATH) == 0)) {
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errno = EBADF;
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return -1;
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}
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char buf[40];
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snprintf(buf, sizeof(buf), "/proc/self/fd/%d", fd);
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errno = saved_errno;
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result = chmod(buf, mode);
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if ((result == -1) && (errno == ELOOP)) {
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// Linux does not support changing the mode of a symlink.
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// For fchmodat(AT_SYMLINK_NOFOLLOW), POSIX requires a return
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// value of ENOTSUP. Assume that's true here too.
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errno = ENOTSUP;
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}
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return result;
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}
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