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authorMaciej W. Rozycki <macro@orcam.me.uk>2021-05-13 11:51:50 +0200
committerLinus Torvalds <torvalds@linux-foundation.org>2021-05-13 11:00:18 -0700
commit860dafa902595fb5f1d23bbcce1215188c3341e6 (patch)
tree6f79ab54104ea46e9014606c3c2f262d3c74228f /drivers/tty
parenta90c275eb144c1b755f04769e1f29d832d6daeaf (diff)
vt: Fix character height handling with VT_RESIZEX
Restore the original intent of the VT_RESIZEX ioctl's `v_clin' parameter which is the number of pixel rows per character (cell) rather than the height of the font used. For framebuffer devices the two values are always the same, because the former is inferred from the latter one. For VGA used as a true text mode device these two parameters are independent from each other: the number of pixel rows per character is set in the CRT controller, while font height is in fact hardwired to 32 pixel rows and fonts of heights below that value are handled by padding their data with blanks when loaded to hardware for use by the character generator. One can change the setting in the CRT controller and it will update the screen contents accordingly regardless of the font loaded. The `v_clin' parameter is used by the `vgacon' driver to set the height of the character cell and then the cursor position within. Make the parameter explicit then, by defining a new `vc_cell_height' struct member of `vc_data', set it instead of `vc_font.height' from `v_clin' in the VT_RESIZEX ioctl, and then use it throughout the `vgacon' driver except where actual font data is accessed which as noted above is independent from the CRTC setting. This way the framebuffer console driver is free to ignore the `v_clin' parameter as irrelevant, as it always should have, avoiding any issues attempts to give the parameter a meaning there could have caused, such as one that has led to commit 988d0763361b ("vt_ioctl: make VT_RESIZEX behave like VT_RESIZE"): "syzbot is reporting UAF/OOB read at bit_putcs()/soft_cursor() [1][2], for vt_resizex() from ioctl(VT_RESIZEX) allows setting font height larger than actual font height calculated by con_font_set() from ioctl(PIO_FONT). Since fbcon_set_font() from con_font_set() allocates minimal amount of memory based on actual font height calculated by con_font_set(), use of vt_resizex() can cause UAF/OOB read for font data." The problem first appeared around Linux 2.5.66 which predates our repo history, but the origin could be identified with the old MIPS/Linux repo also at: <git://git.kernel.org/pub/scm/linux/kernel/git/ralf/linux.git> as commit 9736a3546de7 ("Merge with Linux 2.5.66."), where VT_RESIZEX code in `vt_ioctl' was updated as follows: if (clin) - video_font_height = clin; + vc->vc_font.height = clin; making the parameter apply to framebuffer devices as well, perhaps due to the use of "font" in the name of the original `video_font_height' variable. Use "cell" in the new struct member then to avoid ambiguity. References: [1] https://syzkaller.appspot.com/bug?id=32577e96d88447ded2d3b76d71254fb855245837 [2] https://syzkaller.appspot.com/bug?id=6b8355d27b2b94fb5cedf4655e3a59162d9e48e3 Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Cc: stable@vger.kernel.org # v2.6.12+ Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'drivers/tty')
-rw-r--r--drivers/tty/vt/vt_ioctl.c6
1 files changed, 3 insertions, 3 deletions
diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
index 95d10197566b..0e0cd9e9e589 100644
--- a/drivers/tty/vt/vt_ioctl.c
+++ b/drivers/tty/vt/vt_ioctl.c
@@ -706,17 +706,17 @@ static int vt_resizex(struct vc_data *vc, struct vt_consize __user *cs)
if (vcp) {
int ret;
int save_scan_lines = vcp->vc_scan_lines;
- int save_font_height = vcp->vc_font.height;
+ int save_cell_height = vcp->vc_cell_height;
if (v.v_vlin)
vcp->vc_scan_lines = v.v_vlin;
if (v.v_clin)
- vcp->vc_font.height = v.v_clin;
+ vcp->vc_cell_height = v.v_clin;
vcp->vc_resize_user = 1;
ret = vc_resize(vcp, v.v_cols, v.v_rows);
if (ret) {
vcp->vc_scan_lines = save_scan_lines;
- vcp->vc_font.height = save_font_height;
+ vcp->vc_cell_height = save_cell_height;
console_unlock();
return ret;
}