Re: binfmts.h MAX_ARG_STRINGS excessive value allows heap spraying
From: Carlos O'Donell <hidden>
Date: 2017-03-09 20:34:37
On 03/09/2017 09:14 AM, Leonard den Ottolander wrote:
quoted hunk ↗ jump to hunk
--- a/include/uapi/linux/binfmts.h 2016-11-23 21:02:31.000000000 +0100 +++ b/include/uapi/linux/binfmts.h 2017-03-09 01:52:14.716319950 +0100@@ -9,10 +9,15 @@ struct pt_regs; * These are the maximum length and maximum number of strings passed to the * execve() system call. MAX_ARG_STRLEN is essentially random but serves to * prevent the kernel from being unduly impacted by misaddressed pointers. - * MAX_ARG_STRINGS is chosen to fit in a signed 32-bit integer. + * MAX_ARG_STRINGS * MAX_ARG_STRLEN should be smaller than the 4GiB + * address space on 32 bit to avoid heap spraying.
OK
+ * MAX_ARG_STRSLEN to the rescue - the MAX_ARG_PAGES concept was there + * for a reason.
Why not just use MAX_ARG_PAGES? Define it in include/linux/binfmts.h for !CONFIG_MMU case.
+ * We can now safely increase STRINGS * STRLEN beyond 4GiB if need be.
*/ -#define MAX_ARG_STRLEN (PAGE_SIZE * 32) -#define MAX_ARG_STRINGS 0x7FFFFFFF +#define MAX_ARG_STRSLEN 262144
You don't need this if you define MAX_ARG_PAGES.
+#define MAX_ARG_STRLEN 65536 +#define MAX_ARG_STRINGS 4096
These should be left untouched at their original values.
quoted hunk ↗ jump to hunk
/* sizeof(linux_binprm->buf) */ #define BINPRM_BUF_SIZE 128--- a/fs/exec.c 2017-02-20 08:04:43.000000000 +0100 +++ b/fs/exec.c 2017-03-09 01:54:25.931476370 +0100@@ -459,6 +459,7 @@ static int copy_strings(int argc, struct char *kaddr = NULL; unsigned long kpos = 0; int ret; + int total_bytes = 0; while (argc-- > 0) { const char __user *str;@@ -478,6 +479,11 @@ static int copy_strings(int argc, struct if (!valid_arg_len(bprm, len)) goto out; + /* -E2BIG is fine for now */ + total_bytes += len; + if (total_bytes > MAX_ARG_STRSLEN)
Should be PAGE_SIZE * MAX_ARG_PAGES, similar to the !CONFIG_MMU case.
+ goto out; + /* We're going to work our way backwords. */ pos = bprm->p; str += len; I've successfully built a kernel on a system with a kernel using above values. As we have now introduced a safeguard (MAX_ARG_STRSLEN capping the total amount of memory reserved) the values of MAX_ARG_STRLEN and MAX_ARG_STRINGS can be increased beyond where their multiplication reaches 2^32. So if we really want to support lets say users having directories of 128k files we can now safely set MAX_ARG_STRINGS to 131072 and assuming an average file name length of 32 set MAX_ARG_STRSLEN to 4194304. Seems a little excessive to me for a default, but it is now safe.
We're getting closer to a solution. There is still no justification for the value of MAX_ARG_PAGES though. My objection that build systems will break still stands. The point of memory is to be used. A reasonable limit might be 1/4 of the virtual address space used for argument pages though. -- Cheers, Carlos.