Re: [PATCH 6.12 000/565] 6.12.58-rc1 review

[Jari Ruusu]

On Friday, November 21st, 2025 at 11:36, Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Nov 11, 2025 at 06:38:21AM +0000, Jari Ruusu wrote:
> > Backporting upstream fixes that use guard() locking to older stable
> > branches that use old-school locking need "extra sports".
> > 
> > Please consider dropping or fixing above mentioned patch.
> 
> Can you provide a fixed up patch?

Below is small test program to poke /dev/console using
ioctl(VT_RESIZE) with deliberately bad parameters that
should trigger failed return status. Opening /dev/console
needs appropriate privileges.

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <linux/vt.h>
#include <sys/ioctl.h>
int main(int argc, char **argv)
{
  int f, x, r;
  struct vt_sizes s = { 40000, 40000, 0 };
  if((f = open("/dev/console", O_RDONLY)) < 0) {
    printf("open() failed\n");
    exit(1);
  }
  for(x = 1; x <= 3; x++) {
    printf("Starting ioctl(VT_RESIZE), try %d\n", x);
    r = ioctl(f, VT_RESIZE, &s);
    printf(" got status %d (expected -1)\n", r);
  }
  close(f);
  exit(0);
}

On kernels that return proper failed status,
it plays like this:

| Starting ioctl(VT_RESIZE), try 1
|  got status -1 (expected -1)
| Starting ioctl(VT_RESIZE), try 2
|  got status -1 (expected -1)
| Starting ioctl(VT_RESIZE), try 3
|  got status -1 (expected -1)

On kernels that return "incorrect success" status,
it plays like this:

| Starting ioctl(VT_RESIZE), try 1
|  got status 0 (expected -1)
| Starting ioctl(VT_RESIZE), try 2
|  got status 0 (expected -1)
| Starting ioctl(VT_RESIZE), try 3
|  got status 0 (expected -1)
   
On kernels that return proper failed status but with
messed up console locking in error handling code path,
it locks up like this:

| Starting ioctl(VT_RESIZE), try 1
|  got status -1 (expected -1)
| Starting ioctl(VT_RESIZE), try 2

After that, the console is FUBAR.

Below is a patch for 6.12.58+ and 6.17.8+ stable branches only.
Upstream does not need this.
Signed-off-by: Jari Ruusu <jariruusu@xxxxxxxxxxxxxx>
Fixes: da7e8b382396 ("tty/vt: Add missing return value for VT_RESIZE in vt_ioctl()")

--- a/drivers/tty/vt/vt_ioctl.c
+++ b/drivers/tty/vt/vt_ioctl.c
@@ -924,8 +924,10 @@
 			if (vc) {
 				/* FIXME: review v tty lock */
 				ret = __vc_resize(vc_cons[i].d, cc, ll, true);
-				if (ret)
+				if (ret) {
+					console_unlock();
 					return ret;
+				}
 			}
 		}
 		console_unlock();

--
Jari Ruusu  4096R/8132F189 12D6 4C3A DCDA 0AA4 27BD  ACDF F073 3C80 8132 F189