[PATCH] audit: fix a double fetch in audit_log_single_execve_arg()

From: Paul Moore <paul(a)paul-moore.com&gt; There is a double fetch problem in audit_log_single_execve_arg() where we first check the execve(2) argumnets for any "bad" characters which would require hex encoding and then re-fetch the arguments for logging in the audit record[1]. Of course this leaves a window of opportunity for an unsavory application to munge with the data. This patch reworks things by only fetching the argument data once[2] into a buffer where it is scanned and logged into the audit records(s). In addition to fixing the double fetch, this patch improves on the original code in a few other ways: better handling of large arguments which require encoding, stricter record length checking, and some performance improvements (completely unverified, but we got rid of some strlen() calls, that's got to be a good thing). As part of the development of this patch, I've also created a basic regression test for the audit-testsuite, the test can be tracked on GitHub at the following link: * https://github.com/linux-audit/audit-testsuite/issues/25 [1] If you pay careful attention, there is actually a triple fetch problem due to a strnlen_user() call at the top of the function. [2] This is a tiny white lie, we do make a call to strnlen_user() prior to fetching the argument data. I don't like it, but due to the way the audit record is structured we really have no choice unless we copy the entire argument at once (which would require a rather wasteful allocation). The good news is that with this patch the kernel no longer relies on this strnlen_user() value for anything beyond recording it in the log, we also update it with a trustworthy value whenever possible. Reported-by: Pengfei Wang <wpengfeinudt(a)gmail.com&gt; Cc: <stable(a)vger.kernel.org&gt; Signed-off-by: Paul Moore <paul(a)paul-moore.com&gt; --- kernel/auditsc.c | 332 +++++++++++++++++++++++++++--------------------------- 1 file changed, 164 insertions(+), 168 deletions(-) diff --git a/kernel/auditsc.c b/kernel/auditsc.c index aa3feec..c65af21 100644 --- a/kernel/auditsc.c +++ b/kernel/auditsc.c @@ -73,6 +73,7 @@ #include <linux/compat.h> #include <linux/ctype.h> #include <linux/string.h> +#include <linux/uaccess.h> #include <uapi/linux/limits.h> #include "audit.h" @@ -82,7 +83,8 @@ #define AUDITSC_SUCCESS 1 #define AUDITSC_FAILURE 2 -/* no execve audit message should be longer than this (userspace limits) */ +/* no execve audit message should be longer than this (userspace limits), + * see the note near the top of audit_log_execve_info() about this value */ #define MAX_EXECVE_AUDIT_LEN 7500 /* max length to print of cmdline/proctitle value during audit */ @@ -992,184 +994,178 @@ static int audit_log_pid_context(struct audit_context *context, pid_t pid, return rc; } -/* - * to_send and len_sent accounting are very loose estimates. We aren't - * really worried about a hard cap to MAX_EXECVE_AUDIT_LEN so much as being - * within about 500 bytes (next page boundary) - * - * why snprintf? an int is up to 12 digits long. if we just assumed when - * logging that a[%d]= was going to be 16 characters long we would be wasting - * space in every audit message. In one 7500 byte message we can log up to - * about 1000 min size arguments. That comes down to about 50% waste of space - * if we didn't do the snprintf to find out how long arg_num_len was. - */ -static int audit_log_single_execve_arg(struct audit_context *context, - struct audit_buffer **ab, - int arg_num, - size_t *len_sent, - const char __user *p, - char *buf) +static void audit_log_execve_info(struct audit_context *context, + struct audit_buffer **ab) { - char arg_num_len_buf[12]; - const char __user *tmp_p = p; - /* how many digits are in arg_num? 5 is the length of ' a=""' */ - size_t arg_num_len = snprintf(arg_num_len_buf, 12, "%d", arg_num) + 5; - size_t len, len_left, to_send; - size_t max_execve_audit_len = MAX_EXECVE_AUDIT_LEN; - unsigned int i, has_cntl = 0, too_long = 0; - int ret; - - /* strnlen_user includes the null we don't want to send */ - len_left = len = strnlen_user(p, MAX_ARG_STRLEN) - 1; - - /* - * We just created this mm, if we can't find the strings - * we just copied into it something is _very_ wrong. Similar - * for strings that are too long, we should not have created - * any. - */ - if (WARN_ON_ONCE(len < 0 || len > MAX_ARG_STRLEN - 1)) { - send_sig(SIGKILL, current, 0); - return -1; + long len_max; + long len_rem; + long len_full; + long len_buf; + long len_abuf; + long len_tmp; + bool require_data; + bool encode; + unsigned int iter; + unsigned int arg; + char *buf_head; + char *buf; + const char __user *p = (const char __user *)current->mm->arg_start; + + /* NOTE: this buffer needs to be large enough to hold all the non-arg + * data we put in the audit record for this argument (see the + * code below) ... at this point in time 96 is plenty */ + char abuf[96]; + + /* NOTE: we set MAX_EXECVE_AUDIT_LEN to a rather arbitrary limit, the + * current value of 7500 is not as important as the fact that it + * is less than 8k, a setting of 7500 gives us plenty of wiggle + * room if we go over a little bit in the logging below */ + WARN_ON_ONCE(MAX_EXECVE_AUDIT_LEN > 7500); + len_max = MAX_EXECVE_AUDIT_LEN; + + /* scratch buffer to hold the userspace args */ + buf_head = kmalloc(MAX_EXECVE_AUDIT_LEN + 1, GFP_KERNEL); + if (!buf_head) { + audit_panic("out of memory for argv string"); + return; } + buf = buf_head; - /* walk the whole argument looking for non-ascii chars */ + audit_log_format(*ab, "argc=%d", context->execve.argc); + + len_rem = len_max; + len_buf = 0; + len_full = 0; + require_data = true; + encode = false; + iter = 0; + arg = 0; do { - if (len_left > MAX_EXECVE_AUDIT_LEN) - to_send = MAX_EXECVE_AUDIT_LEN; - else - to_send = len_left; - ret = copy_from_user(buf, tmp_p, to_send); - /* - * There is no reason for this copy to be short. We just - * copied them here, and the mm hasn't been exposed to user- - * space yet. - */ - if (ret) { - WARN_ON(1); - send_sig(SIGKILL, current, 0); - return -1; - } - buf[to_send] = '\0'; - has_cntl = audit_string_contains_control(buf, to_send); - if (has_cntl) { - /* - * hex messages get logged as 2 bytes, so we can only - * send half as much in each message - */ - max_execve_audit_len = MAX_EXECVE_AUDIT_LEN / 2; - break; - } - len_left -= to_send; - tmp_p += to_send; - } while (len_left > 0); - - len_left = len; - - if (len > max_execve_audit_len) - too_long = 1; - - /* rewalk the argument actually logging the message */ - for (i = 0; len_left > 0; i++) { - int room_left; - - if (len_left > max_execve_audit_len) - to_send = max_execve_audit_len; - else - to_send = len_left; - - /* do we have space left to send this argument in this ab? */ - room_left = MAX_EXECVE_AUDIT_LEN - arg_num_len - *len_sent; - if (has_cntl) - room_left -= (to_send * 2); - else - room_left -= to_send; - if (room_left < 0) { - *len_sent = 0; - audit_log_end(*ab); - *ab = audit_log_start(context, GFP_KERNEL, AUDIT_EXECVE); - if (!*ab) - return 0; - } + /* NOTE: we don't ever want to trust this value for anything + * serious, but the audit record format insists we + * provide an argument length for really long arguments, + * e.g. > MAX_EXECVE_AUDIT_LEN, so we have no choice but + * to use strncpy_from_user() to obtain this value for + * recording in the log, although we don't use it + * anywhere here to avoid a double-fetch problem */ + if (len_full == 0) + len_full = strnlen_user(p, MAX_ARG_STRLEN) - 1; + + /* read more data from userspace */ + if (require_data) { + /* can we make more room in the buffer? */ + if (buf != buf_head) { + memmove(buf_head, buf, len_buf); + buf = buf_head; + } + + /* fetch as much as we can of the argument */ + len_tmp = strncpy_from_user(&buf_head[len_buf], p, + len_max - len_buf); + if (len_tmp == -EFAULT) { + /* unable to copy from userspace */ + send_sig(SIGKILL, current, 0); + goto out; + } else if (len_tmp == (len_max - len_buf)) { + /* buffer is not large enough */ + require_data = true; + /* NOTE: if we are going to span multiple + * buffers force the encoding so we stand + * a chance at a sane len_full value and + * consistent record encoding */ + encode = true; + len_full = len_full * 2; + p += len_tmp; + } else { + require_data = false; + if (!encode) + encode = audit_string_contains_control( + buf, len_tmp); + /* try to use a trusted value for len_full */ + if (len_full < len_max) + len_full = (encode ? + len_tmp * 2 : len_tmp); + p += len_tmp + 1; + } + len_buf += len_tmp; + buf_head[len_buf] = '\0'; - /* - * first record needs to say how long the original string was - * so we can be sure nothing was lost. - */ - if ((i == 0) && (too_long)) - audit_log_format(*ab, " a%d_len=%zu", arg_num, - has_cntl ? 2*len : len); - - /* - * normally arguments are small enough to fit and we already - * filled buf above when we checked for control characters - * so don't bother with another copy_from_user - */ - if (len >= max_execve_audit_len) - ret = copy_from_user(buf, p, to_send); - else - ret = 0; - if (ret) { - WARN_ON(1); - send_sig(SIGKILL, current, 0); - return -1; + /* length of the buffer in the audit record? */ + len_abuf = (encode ? len_buf * 2 : len_buf + 2); } - buf[to_send] = '\0'; - - /* actually log it */ - audit_log_format(*ab, " a%d", arg_num); - if (too_long) - audit_log_format(*ab, "[%d]", i); - audit_log_format(*ab, "="); - if (has_cntl) - audit_log_n_hex(*ab, buf, to_send); - else - audit_log_string(*ab, buf); - - p += to_send; - len_left -= to_send; - *len_sent += arg_num_len; - if (has_cntl) - *len_sent += to_send * 2; - else - *len_sent += to_send; - } - /* include the null we didn't log */ - return len + 1; -} -static void audit_log_execve_info(struct audit_context *context, - struct audit_buffer **ab) -{ - int i, len; - size_t len_sent = 0; - const char __user *p; - char *buf; + /* write as much as we can to the audit log */ + if (len_buf > 0) { + /* NOTE: some magic numbers here - basically if we + * can't fit a reasonable amount of data into the + * existing audit buffer, flush it and start with + * a new buffer */ + if ((sizeof(abuf) + 8) > len_rem) { + len_rem = len_max; + audit_log_end(*ab); + *ab = audit_log_start(context, + GFP_KERNEL, AUDIT_EXECVE); + if (!*ab) + goto out; + } - p = (const char __user *)current->mm->arg_start; + /* create the non-arg portion of the arg record */ + len_tmp = 0; + if (require_data || (iter > 0) || + ((len_abuf + sizeof(abuf)) > len_rem)) { + if (iter == 0) { + len_tmp += snprintf(&abuf[len_tmp], + sizeof(abuf) - len_tmp, + " a%d_len=%lu", + arg, len_full); + } + len_tmp += snprintf(&abuf[len_tmp], + sizeof(abuf) - len_tmp, + " a%d[%d]=", arg, iter++); + } else + len_tmp += snprintf(&abuf[len_tmp], + sizeof(abuf) - len_tmp, + " a%d=", arg); + WARN_ON(len_tmp >= sizeof(abuf)); + abuf[sizeof(abuf) - 1] = '\0'; + + /* log the arg in the audit record */ + audit_log_format(*ab, "%s", abuf); + len_rem -= len_tmp; + len_tmp = len_buf; + if (encode) { + if (len_abuf > len_rem) + len_tmp = len_rem / 2; /* encoding */ + audit_log_n_hex(*ab, buf, len_tmp); + len_rem -= len_tmp * 2; + len_abuf -= len_tmp * 2; + } else { + if (len_abuf > len_rem) + len_tmp = len_rem - 2; /* quotes */ + audit_log_n_string(*ab, buf, len_tmp); + len_rem -= len_tmp + 2; + /* don't subtract the "2" because we still need + * to add quotes to the remaining string */ + len_abuf -= len_tmp; + } + len_buf -= len_tmp; + buf += len_tmp; + } - audit_log_format(*ab, "argc=%d", context->execve.argc); + /* ready to move to the next argument? */ + if ((len_buf == 0) && !require_data) { + arg++; + iter = 0; + len_full = 0; + require_data = true; + encode = false; + } + } while (arg < context->execve.argc); - /* - * we need some kernel buffer to hold the userspace args. Just - * allocate one big one rather than allocating one of the right size - * for every single argument inside audit_log_single_execve_arg() - * should be <8k allocation so should be pretty safe. - */ - buf = kmalloc(MAX_EXECVE_AUDIT_LEN + 1, GFP_KERNEL); - if (!buf) { - audit_panic("out of memory for argv string"); - return; - } + /* NOTE: the caller handles the final audit_log_end() call */ - for (i = 0; i < context->execve.argc; i++) { - len = audit_log_single_execve_arg(context, ab, i, - &len_sent, p, buf); - if (len <= 0) - break; - p += len; - } - kfree(buf); +out: + kfree(buf_head); } static void show_special(struct audit_context *context, int *call_panic)