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SubjectRe: [PATCH] Fix user data corrupted by old value return of sysctl
On Fri, Dec 30, 2005 at 09:25:35AM -0800, Linus Torvalds wrote:
> On Fri, 30 Dec 2005, Yi Yang wrote:
> >
> > If the user reads a sysctl entry which is of string type
> > by sysctl syscall, this call probably corrupts the user data
> > right after the old value buffer, the issue lies in sysctl_string
> > seting 0 to oldval[len], len is the available buffer size
> > specified by the user, obviously, this will write to the first
> > byte of the user memory place immediate after the old value buffer,
> > the correct way is that sysctl_string doesn't set 0, the user
> > should do it by self in the program.
> Hmm.. I think this patch is incomplete.
> We _should_ zero-pad the data, at least if the result fits in the buffer.
> So I think the correct fix is to just _copy_ the last zero if it fits in
> the buffer, rather than do the unconditional "add NUL at the end" thing.
> The simplest way to do that is to just make "l" be "strlen(str)+1", so
> that we count the ending NUL in the length (and then, if the buffer isn't
> big enough, we will truncate it).
> In other words, I would instead suggest a patch like the appended.
> But even that is questionable: one alternative is to always zero-pad (like
> we used to), but make sure that the buffer size is sufficient for it (ie
> instead of adding one to the length of the string, we'd subtract one from
> the buffer length and make sure that the '\0' fits..
> Comments?

Always do zero-pad please. I'd feel more comfortable with C strings (NULL
terminated). Don't you?

Poor code, also a small cleanup attached.

Signed-off-by: Coywolf Qi Hunt <>

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index e5102ea..9960a26 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2203,7 +2203,7 @@ int sysctl_string(ctl_table *table, int
if (len) {
l = strlen(table->data)+1;
if (len > l) len = l;
- if (len >= table->maxlen)
+ if (len > table->maxlen)
len = table->maxlen;
if(copy_to_user(oldval, table->data, len))
return -EFAULT;
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