Re: [PATCH] audit: Fix check of return value of strnlen_user()

On Thu, Jun 4, 2015 at 3:36 AM, Jan Kara <jack(a)suse.cz&gt; wrote: > On Wed 03-06-15 14:56:18, Paul Moore wrote: >> On Tuesday, June 02, 2015 05:08:29 PM Jan Kara wrote: >> > strnlen_user() returns 0 when it hits fault, not -1. Fix the test in >> > audit_log_single_execve_arg(). Luckily this shouldn't ever happen unless >> > there's a kernel bug so it's mostly a cosmetic fix. >> > >> > CC: Paul Moore <pmoore(a)redhat.com&gt; >> > Signed-off-by: Jan Kara <jack(a)suse.cz&gt; >> > --- >> > kernel/auditsc.c | 2 +- >> > 1 file changed, 1 insertion(+), 1 deletion(-) >> > >> > diff --git a/kernel/auditsc.c b/kernel/auditsc.c >> > index 9fb9d1cb83ce..bb947ceeee4d 100644 >> > --- a/kernel/auditsc.c >> > +++ b/kernel/auditsc.c >> > @@ -1023,7 +1023,7 @@ static int audit_log_single_execve_arg(struct >> > audit_context *context, * for strings that are too long, we should not have >> > created >> > * any. >> > */ >> > - if (unlikely((len == -1) || len > MAX_ARG_STRLEN - 1)) { >> > + if (unlikely((len == 0) || len > MAX_ARG_STRLEN - 1)) { >> >> While we're at it, should we make it just "len > MAX_ARG_STRLEN" as well? >> Reading the comments in include/uapi/linux/binfmts.h as well as >> valid_arg_len() that seems to be the correct logic. > > Umm, but audit_log_single_execve_arg() does decrement 1 from > strnlen_user() result before doing the comparison. So the current test > seems to match the one in valid_arg_len() exactly... For reference (taken from fs/exec.c in Linus' tree just now): static bool valid_arg_len(struct linux_binprm *bprm, long len) { return len <= MAX_ARG_STRLEN; } The valid_arg_len() returns true when the length is less than or equal to MAX_ARG_STRLEN, implying that lengths greater than MAX_ARG_STRLEN are invalid. The existing test in audit_log_single_execve_arg() treats lengths greater than (MAX_ARG_STRLEN-1) as invalid.