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SubjectRe: get_arg_page() && ptr_size accounting
On 09/12, Oleg Nesterov wrote:
>
> it's pity. cause this means I will have to actually test this change and
> (worse) write the changelog ;)

Seems to work, see v2 below, I tried to address your comments.

The new helper is prepare_arg_pages(), matches setup_arg_pages/shift_arg_pages.

s/p_min/argmin/. Or please suggest a better name and/or comment.

I still think the new helper should absorb the bprm->rlim_stack inititialization,
it is called right after bprm_mm_init() and I see no reason why arch_bprm_mm_init()
should ever look at rlim_stack, but this is minor and I won't insist.

If you do not see any problems I'll try to force myself to write the changelog
tomorrow.

Oleg.


diff --git a/fs/exec.c b/fs/exec.c
index 1ebf6e5..9a892ac 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -218,55 +218,10 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
if (ret <= 0)
return NULL;

- if (write) {
- unsigned long size = bprm->vma->vm_end - bprm->vma->vm_start;
- unsigned long ptr_size, limit;
-
- /*
- * Since the stack will hold pointers to the strings, we
- * must account for them as well.
- *
- * The size calculation is the entire vma while each arg page is
- * built, so each time we get here it's calculating how far it
- * is currently (rather than each call being just the newly
- * added size from the arg page). As a result, we need to
- * always add the entire size of the pointers, so that on the
- * last call to get_arg_page() we'll actually have the entire
- * correct size.
- */
- ptr_size = (bprm->argc + bprm->envc) * sizeof(void *);
- if (ptr_size > ULONG_MAX - size)
- goto fail;
- size += ptr_size;
-
- acct_arg_size(bprm, size / PAGE_SIZE);
-
- /*
- * We've historically supported up to 32 pages (ARG_MAX)
- * of argument strings even with small stacks
- */
- if (size <= ARG_MAX)
- return page;
-
- /*
- * Limit to 1/4 of the max stack size or 3/4 of _STK_LIM
- * (whichever is smaller) for the argv+env strings.
- * This ensures that:
- * - the remaining binfmt code will not run out of stack space,
- * - the program will have a reasonable amount of stack left
- * to work from.
- */
- limit = _STK_LIM / 4 * 3;
- limit = min(limit, bprm->rlim_stack.rlim_cur / 4);
- if (size > limit)
- goto fail;
- }
+ if (write)
+ acct_arg_size(bprm, vma_pages(bprm->vma));

return page;
-
-fail:
- put_page(page);
- return NULL;
}

static void put_arg_page(struct page *page)
@@ -492,6 +447,50 @@ static int count(struct user_arg_ptr argv, int max)
return i;
}

+static int prepare_arg_pages(struct linux_binprm *bprm,
+ struct user_arg_ptr argv, struct user_arg_ptr envp)
+{
+ unsigned long limit, ptr_size;
+
+ bprm->argc = count(argv, MAX_ARG_STRINGS);
+ if (bprm->argc < 0)
+ return bprm->argc;
+
+ bprm->envc = count(envp, MAX_ARG_STRINGS);
+ if (bprm->envc < 0)
+ return bprm->envc;
+
+ /*
+ * Limit to 1/4 of the max stack size or 3/4 of _STK_LIM
+ * (whichever is smaller) for the argv+env strings.
+ * This ensures that:
+ * - the remaining binfmt code will not run out of stack space,
+ * - the program will have a reasonable amount of stack left
+ * to work from.
+ */
+ limit = _STK_LIM / 4 * 3;
+ limit = min(limit, bprm->rlim_stack.rlim_cur / 4);
+ /*
+ * We've historically supported up to 32 pages (ARG_MAX)
+ * of argument strings even with small stacks
+ */
+ limit = max(limit, (unsigned long)ARG_MAX);
+ /*
+ * We must account for the size of all the argv and envp pointers to
+ * the argv and envp strings, since they will also take up space in
+ * the stack. They aren't stored until much later when we can't
+ * signal to the parent that the child has run out of stack space.
+ * Instead, calculate it here so it's possible to fail gracefully.
+ */
+ ptr_size = (bprm->argc + bprm->envc) * sizeof(void *);
+ if (limit <= ptr_size)
+ return -E2BIG;
+ limit -= ptr_size;
+
+ bprm->argmin = bprm->p - limit;
+ return 0;
+}
+
/*
* 'copy_strings()' copies argument/environment strings from the old
* processes's memory to the new process's stack. The call to get_user_pages()
@@ -527,6 +526,8 @@ static int copy_strings(int argc, struct user_arg_ptr argv,
pos = bprm->p;
str += len;
bprm->p -= len;
+ if (bprm->p < bprm->argmin)
+ goto out;

while (len > 0) {
int offset, bytes_to_copy;
@@ -1789,12 +1790,8 @@ static int __do_execve_file(int fd, struct filename *filename,
if (retval)
goto out_unmark;

- bprm->argc = count(argv, MAX_ARG_STRINGS);
- if ((retval = bprm->argc) < 0)
- goto out;
-
- bprm->envc = count(envp, MAX_ARG_STRINGS);
- if ((retval = bprm->envc) < 0)
+ retval = prepare_arg_pages(bprm, argv, envp);
+ if (retval < 0)
goto out;

retval = prepare_binprm(bprm);
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index c05f24f..5ffc8da 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -25,6 +25,7 @@ struct linux_binprm {
#endif
struct mm_struct *mm;
unsigned long p; /* current top of mem */
+ unsigned long argmin; /* rlimit marker for copy_strings() */
unsigned int
/*
* True after the bprm_set_creds hook has been called once
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