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kernel: make exec clear errstr, stop side-channels and truncate on utf8 boundary

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make exec() clear the per process error string
to avoid spurious errors and confusion.

the errstr() syscall used to always swap the
maximum buffer size with memmove(), which is
problematic as this gives access to the garbage
beyond the NUL byte. worse, newproc(), werrstr()
and rerrstr() only clear the first byte of the
input buffer. so random stack rubble could be
leaked across processes.

we change the errstr() syscall to not copy
beyond the NUL byte.

the manpage also documents that errstr() should
truncate on a utf8 boundary so we use utfecpy()
to ensure proper NUL termination.
This commit is contained in:
cinap_lenrek 2019-09-04 02:40:41 +02:00
parent e4a57c8b8a
commit 6aa7ebcf49
2 changed files with 14 additions and 9 deletions
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1
sys/src/9/port/lib.h
View file

@ -47,6 +47,7 @@ enum
*/
extern int runetochar(char*, Rune*);
extern int chartorune(Rune*, char*);
extern char* utfecpy(char *s1, char *es1, char *s2);
extern char* utfrune(char*, long);
extern int utflen(char*);
extern int utfnlen(char*, long);

22
sys/src/9/port/sysproc.c
View file

@ -572,6 +572,9 @@ sysexec(va_list list)
procsetup(up);
qunlock(&up->debug);
up->errbuf0[0] = '\0';
up->errbuf1[0] = '\0';
/*
* At this point, the mmu contains info about the old address
* space and needs to be flushed
@ -698,20 +701,21 @@ werrstr(char *fmt, ...)
static int
generrstr(char *buf, uint nbuf)
{
char tmp[ERRMAX];
char *err;
if(nbuf == 0)
error(Ebadarg);
if(nbuf > ERRMAX)
nbuf = ERRMAX;
validaddr((uintptr)buf, nbuf, 1);
if(nbuf > sizeof tmp)
nbuf = sizeof tmp;
memmove(tmp, buf, nbuf);
/* make sure it's NUL-terminated */
tmp[nbuf-1] = '\0';
memmove(buf, up->syserrstr, nbuf);
buf[nbuf-1] = '\0';
memmove(up->syserrstr, tmp, nbuf);
err = up->errstr;
utfecpy(err, err+nbuf, buf);
utfecpy(buf, buf+nbuf, up->syserrstr);
up->errstr = up->syserrstr;
up->syserrstr = err;
return 0;
}