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SubjectRe: [PATCH] Fix user data corrupted by old value return of sysctl
Coywolf Qi Hunt wrote:

>2005/12/30, Yi Yang <yang.y.yi@gmail.com>:
>
>
>>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.
>>
>>The following program verifies this point:
>>
>>#include <linux/unistd.h>
>>#include <linux/types.h>
>>#include <linux/sysctl.h>
>>#include <errno.h>
>>
>>_syscall1(int, _sysctl, struct __sysctl_args *, args);
>>int sysctl(int *name, int nlen, void *oldval, size_t *oldlenp,
>> void *newval, size_t newlen)
>>{
>> struct __sysctl_args args
>> = {name,nlen,oldval,oldlenp,newval,newlen};
>>
>> return _sysctl(&args);
>>}
>>
>>#define SIZE(x) sizeof(x)/sizeof(x[0])
>>#define OSNAMESZ 4
>>
>>struct mystruct {
>> char osname[OSNAMESZ];
>> int target;
>> int osnamelth;
>>} myos;
>>
>>int name[] = { CTL_KERN, KERN_NODENAME };
>>
>>int main(int argc, char * argv[])
>>{
>> myos.target = 1;
>> printf("target = %d\n", myos.target);
>> myos.osnamelth = SIZE(myos.osname);
>> if (sysctl(name, SIZE(name), myos.osname,
>> &myos.osnamelth, 0, 0))
>> perror("sysctl");
>> else {
>> printf("Current host name: %s\n", myos.osname);
>> }
>> printf("target = %d\n", myos.target);
>> return 0;
>>}
>>
>>Copy it to file sysctl-safe.c, then
>>$ hostname
>>mylocalmachine
>>$ gcc sysctl-safe.c
>>$ ./a.out
>>target = 1
>>Current host name: mylo
>>target = 0
>>$
>>
>>After apply this patch:
>>$ hostname
>>mylocalmachine
>>$ gcc sysctl-safe.c
>>$ ./a.out
>>target = 1
>>Current host name: mylo
>>
>>
>
>You didn't set the trailing '\0', I wonder how your printf did work
>properly ever. You've just been lucky or something.
>
>-- Coywolf
>
>
The variable target does it, its value is 0x00000001, so you mustn't
worry it.
osname only has 4-bytes space, so if you set '\0' to its tail, a byte
information will be lost.

>
>
>
>>target = 1
>>
>>Signed-off-by: Yi Yang <yang.y.yi@gmail.com>
>>
>>
>>--- a/kernel/sysctl.c.orig 2005-12-30 09:21:34.000000000 +0000
>>+++ b/kernel/sysctl.c 2005-12-30 15:58:15.000000000 +0000
>>@@ -2207,8 +2207,6 @@ int sysctl_string(ctl_table *table, int
>> len = table->maxlen;
>> if(copy_to_user(oldval, table->data, len))
>> return -EFAULT;
>>- if(put_user(0, ((char __user *) oldval) + len))
>>- return -EFAULT;
>> if(put_user(len, oldlenp))
>> return -EFAULT;
>> }
>>
>>
>>
>>
>>
>--
>Coywolf Qi Hunt
>
>
>

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