Hi all, syzbot has reported [1] a global out-of-bounds read issue in fbcon_get_font(). A malicious user may resize `vc_font.height` to a large value in vt_ioctl(), causing fbcon_get_font() to overflow our built-in font data buffers, declared in lib/fonts/font_*.c: (e.g. lib/fonts/font_8x8.c) #define FONTDATAMAX 2048 static const unsigned char fontdata_8x8[FONTDATAMAX] = { /* 0 0x00 '^@' */ 0x00, /* 00000000 */ 0x00, /* 00000000 */ 0x00, /* 00000000 */ 0x00, /* 00000000 */ 0x00, /* 00000000 */ 0x00, /* 00000000 */ 0x00, /* 00000000 */ 0x00, /* 00000000 */ [...] In order to perform a reliable range check, fbcon_get_font() needs to know `FONTDATAMAX` for each built-in font under lib/fonts/. Unfortunately, we do not keep that information in our font descriptor, `struct console_font`: (include/uapi/linux/kd.h) struct console_font { unsigned int width, height; /* font size */ unsigned int charcount; unsigned char *data; /* font data with height fixed to 32 */ }; To make things worse, `struct console_font` is part of the UAPI, so we cannot add a new field to keep track of `FONTDATAMAX`. Fortunately, the framebuffer layer itself gives us a hint of how to resolve this issue without changing UAPI. When allocating a buffer for a user-provided font, fbcon_set_font() reserves four "extra words" at the beginning of the buffer: (drivers/video/fbdev/core/fbcon.c) new_data = kmalloc(FONT_EXTRA_WORDS * sizeof(int) + size, GFP_USER); [...] new_data += FONT_EXTRA_WORDS * sizeof(int); FNTSIZE(new_data) = size; FNTCHARCNT(new_data) = charcount; REFCOUNT(new_data) = 0; /* usage counter */ [...] FNTSUM(new_data) = csum; Later, to get the size of a data buffer, the framebuffer layer simply calls FNTSIZE() on it: (drivers/video/fbdev/core/fbcon.h) /* Font */ #define REFCOUNT(fd) (((int *)(fd))[-1]) #define FNTSIZE(fd) (((int *)(fd))[-2]) #define FNTCHARCNT(fd) (((int *)(fd))[-3]) #define FNTSUM(fd) (((int *)(fd))[-4]) #define FONT_EXTRA_WORDS 4 Currently, this is only done for user-provided fonts. Let us do the same thing for built-in fonts, prepend these "extra words" (including `FONTDATAMAX`) to their data buffers, so that other subsystems, like the framebuffer layer, can use these macros on all fonts, no matter built-in or user-provided. As an example, this series fixes the syzbot issue in fbcon_get_font(): (drivers/video/fbdev/core/fbcon.c) if (font->width <= 8) { j = vc->vc_font.height; + if (font->charcount * j > FNTSIZE(fontdata)) + return -EINVAL; [...] Similarly, newport_con also use these macros. It only uses three of them: (drivers/video/console/newport_con.c) /* borrowed from fbcon.c */ #define REFCOUNT(fd) (((int *)(fd))[-1]) #define FNTSIZE(fd) (((int *)(fd))[-2]) #define FNTCHARCNT(fd) (((int *)(fd))[-3]) #define FONT_EXTRA_WORDS 3 To keep things simple, move all these macro definitions to <linux/font.h>, use four words instead of three, and initialize the fourth word in newport_set_font() properly. Many thanks to Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>, who reviewed and improved this series! [1]: KASAN: global-out-of-bounds Read in fbcon_get_font https://syzkaller.appspot.com/bug?id=08b8be45afea11888776f897895aef9ad1c3ecfd Peilin Ye (3): fbdev, newport_con: Move FONT_EXTRA_WORDS macros into linux/font.h Fonts: Support FONT_EXTRA_WORDS macros for built-in fonts fbcon: Fix global-out-of-bounds read in fbcon_get_font() drivers/video/console/newport_con.c | 7 +------ drivers/video/fbdev/core/fbcon.c | 12 ++++++++++++ drivers/video/fbdev/core/fbcon.h | 7 ------- drivers/video/fbdev/core/fbcon_rotate.c | 1 + drivers/video/fbdev/core/tileblit.c | 1 + include/linux/font.h | 13 +++++++++++++ lib/fonts/font_10x18.c | 9 ++++----- lib/fonts/font_6x10.c | 9 +++++---- lib/fonts/font_6x11.c | 9 ++++----- lib/fonts/font_7x14.c | 9 ++++----- lib/fonts/font_8x16.c | 9 ++++----- lib/fonts/font_8x8.c | 9 ++++----- lib/fonts/font_acorn_8x8.c | 9 ++++++--- lib/fonts/font_mini_4x6.c | 8 ++++---- lib/fonts/font_pearl_8x8.c | 9 ++++----- lib/fonts/font_sun12x22.c | 9 ++++----- lib/fonts/font_sun8x16.c | 7 ++++--- lib/fonts/font_ter16x32.c | 9 ++++----- 18 files changed, 79 insertions(+), 67 deletions(-) -- 2.25.1 _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel