CVE-2020-28097 (published 2021-06-24T12:15Z)

The vgacon subsystem in the Linux kernel before 5.8.10 mishandles software scrollback. There is a vgacon_scrolldelta out-of-bounds read, aka CID-973c096f6a85.

Severity: LOW
CVSS v2 base score: 3.6 (AV:L/AC:L/Au:N/C:P/I:N/A:P)
CVSS v3 base score: 5.9, MEDIUM (CVSS:3.1/AV:P/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H)

Weakness information:
cwe_id: CWE-125
cwe_name: Out-of-bounds Read
description: The product reads data past the end, or before the beginning, of the intended buffer.
extended_description: 
            
            
             
			 
         
         

Fix information:
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: Linus Torvalds
author_date: 2020-09-09 14:53:50-07:00
msg: vgacon: remove software scrollback support

Yunhai Zhang recently fixed a VGA software scrollback bug in commit
ebfdfeeae8c0 ("vgacon: Fix for missing check in scrollback handling"),
but that then made people look more closely at some of this code, and
there were more problems on the vgacon side, but also the fbcon software
scrollback.

We don't really have anybody who maintains this code - probably because
nobody actually _uses_ it any more.  Sure, people still use both VGA and
the framebuffer consoles, but they are no longer the main user
interfaces to the kernel, and haven't been for decades, so these kinds
of extra features end up bitrotting and not really being used.

So rather than try to maintain a likely unused set of code, I'll just
aggressively remove it, and see if anybody even notices.  Maybe there
are people who haven't jumped on the whole GUI badnwagon yet, and think
it's just a fad.  And maybe those people use the scrollback code.

If that turns out to be the case, we can resurrect this again, once
we've found the sucker^Wmaintainer for it who actually uses it.

Reported-by: NopNop Nop <nopitydays@gmail.com>
Tested-by: Willy Tarreau <w@1wt.eu>
Cc: 张云海 <zhangyunhai@nsfocus.com>
Acked-by: Andy Lutomirski <luto@amacapital.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
repository: repo_url: https://github.com/torvalds/linux
repo_name: torvalds/linux
description: Linux kernel source tree
repo_language: C
owner: torvalds
file_changes: filename: pasemi_defconfig
change_type: ModificationType.MODIFY
diff: @@ -108,7 +108,6 @@ CONFIG_FB_NVIDIA=y
 CONFIG_FB_NVIDIA_I2C=y
 CONFIG_FB_RADEON=y
 # CONFIG_LCD_CLASS_DEVICE is not set
-CONFIG_VGACON_SOFT_SCROLLBACK=y
 CONFIG_LOGO=y
 CONFIG_SOUND=y
 CONFIG_SND=y

programming_language: Shell
filename: ppc6xx_defconfig
change_type: ModificationType.MODIFY
diff: @@ -743,7 +743,6 @@ CONFIG_FB_TRIDENT=m
 CONFIG_FB_SM501=m
 CONFIG_FB_IBM_GXT4500=y
 CONFIG_LCD_PLATFORM=m
-CONFIG_VGACON_SOFT_SCROLLBACK=y
 CONFIG_FRAMEBUFFER_CONSOLE=y
 CONFIG_FRAMEBUFFER_CONSOLE_ROTATION=y
 CONFIG_LOGO=y

programming_language: Shell
filename: i386_defconfig
change_type: ModificationType.MODIFY
diff: @@ -186,7 +186,6 @@ CONFIG_DRM_I915=y
 CONFIG_FB_MODE_HELPERS=y
 CONFIG_FB_TILEBLITTING=y
 CONFIG_FB_EFI=y
-CONFIG_VGACON_SOFT_SCROLLBACK=y
 CONFIG_LOGO=y
 # CONFIG_LOGO_LINUX_MONO is not set
 # CONFIG_LOGO_LINUX_VGA16 is not set

programming_language: Shell
filename: x86_64_defconfig
change_type: ModificationType.MODIFY
diff: @@ -181,7 +181,6 @@ CONFIG_DRM_I915=y
 CONFIG_FB_MODE_HELPERS=y
 CONFIG_FB_TILEBLITTING=y
 CONFIG_FB_EFI=y
-CONFIG_VGACON_SOFT_SCROLLBACK=y
 CONFIG_LOGO=y
 # CONFIG_LOGO_LINUX_MONO is not set
 # CONFIG_LOGO_LINUX_VGA16 is not set

programming_language: Shell
filename: Kconfig
change_type: ModificationType.MODIFY
diff: @@ -22,52 +22,6 @@ config VGA_CONSOLE
 
 	  Say Y.
 
-config VGACON_SOFT_SCROLLBACK
-       bool "Enable Scrollback Buffer in System RAM"
-       depends on VGA_CONSOLE
-       default n
-       help
-	 The scrollback buffer of the standard VGA console is located in
-	 the VGA RAM.  The size of this RAM is fixed and is quite small.
-	 If you require a larger scrollback buffer, this can be placed in
-	 System RAM which is dynamically allocated during initialization.
-	 Placing the scrollback buffer in System RAM will slightly slow
-	 down the console.
-
-	 If you want this feature, say 'Y' here and enter the amount of
-	 RAM to allocate for this buffer.  If unsure, say 'N'.
-
-config VGACON_SOFT_SCROLLBACK_SIZE
-       int "Scrollback Buffer Size (in KB)"
-       depends on VGACON_SOFT_SCROLLBACK
-       range 1 1024
-       default "64"
-       help
-	  Enter the amount of System RAM to allocate for scrollback
-	  buffers of VGA consoles. Each 64KB will give you approximately
-	  16 80x25 screenfuls of scrollback buffer.
-
-config VGACON_SOFT_SCROLLBACK_PERSISTENT_ENABLE_BY_DEFAULT
-	bool "Persistent Scrollback History for each console by default"
-	depends on VGACON_SOFT_SCROLLBACK
-	default n
-	help
-	  Say Y here if the scrollback history should persist by default when
-	  switching between consoles. Otherwise, the scrollback history will be
-	  flushed each time the console is switched. This feature can also be
-	  enabled using the boot command line parameter
-	  'vgacon.scrollback_persistent=1'.
-
-	  This feature might break your tool of choice to flush the scrollback
-	  buffer, e.g. clear(1) will work fine but Debian's clear_console(1)
-	  will be broken, which might cause security issues.
-	  You can use the escape sequence \e[3J instead if this feature is
-	  activated.
-
-	  Note that a buffer of VGACON_SOFT_SCROLLBACK_SIZE is taken for each
-	  created tty device.
-	  So if you use a RAM-constrained system, say N here.
-
 config MDA_CONSOLE
 	depends on !M68K && !PARISC && ISA
 	tristate "MDA text console (dual-headed)"

programming_language: Perl
filename: vgacon.c
change_type: ModificationType.MODIFY
diff: @@ -165,214 +165,6 @@ static inline void vga_set_mem_top(struct vc_data *c)
 	write_vga(12, (c->vc_visible_origin - vga_vram_base) / 2);
 }
 
-#ifdef CONFIG_VGACON_SOFT_SCROLLBACK
-/* software scrollback */
-struct vgacon_scrollback_info {
-	void *data;
-	int tail;
-	int size;
-	int rows;
-	int cnt;
-	int cur;
-	int save;
-	int restore;
-};
-
-static struct vgacon_scrollback_info *vgacon_scrollback_cur;
-static struct vgacon_scrollback_info vgacon_scrollbacks[MAX_NR_CONSOLES];
-static bool scrollback_persistent = \
-	IS_ENABLED(CONFIG_VGACON_SOFT_SCROLLBACK_PERSISTENT_ENABLE_BY_DEFAULT);
-module_param_named(scrollback_persistent, scrollback_persistent, bool, 0000);
-MODULE_PARM_DESC(scrollback_persistent, "Enable persistent scrollback for all vga consoles");
-
-static void vgacon_scrollback_reset(int vc_num, size_t reset_size)
-{
-	struct vgacon_scrollback_info *scrollback = &vgacon_scrollbacks[vc_num];
-
-	if (scrollback->data && reset_size > 0)
-		memset(scrollback->data, 0, reset_size);
-
-	scrollback->cnt  = 0;
-	scrollback->tail = 0;
-	scrollback->cur  = 0;
-}
-
-static void vgacon_scrollback_init(int vc_num)
-{
-	int pitch = vga_video_num_columns * 2;
-	size_t size = CONFIG_VGACON_SOFT_SCROLLBACK_SIZE * 1024;
-	int rows = size / pitch;
-	void *data;
-
-	data = kmalloc_array(CONFIG_VGACON_SOFT_SCROLLBACK_SIZE, 1024,
-			     GFP_NOWAIT);
-
-	vgacon_scrollbacks[vc_num].data = data;
-	vgacon_scrollback_cur = &vgacon_scrollbacks[vc_num];
-
-	vgacon_scrollback_cur->rows = rows - 1;
-	vgacon_scrollback_cur->size = rows * pitch;
-
-	vgacon_scrollback_reset(vc_num, size);
-}
-
-static void vgacon_scrollback_switch(int vc_num)
-{
-	if (!scrollback_persistent)
-		vc_num = 0;
-
-	if (!vgacon_scrollbacks[vc_num].data) {
-		vgacon_scrollback_init(vc_num);
-	} else {
-		if (scrollback_persistent) {
-			vgacon_scrollback_cur = &vgacon_scrollbacks[vc_num];
-		} else {
-			size_t size = CONFIG_VGACON_SOFT_SCROLLBACK_SIZE * 1024;
-
-			vgacon_scrollback_reset(vc_num, size);
-		}
-	}
-}
-
-static void vgacon_scrollback_startup(void)
-{
-	vgacon_scrollback_cur = &vgacon_scrollbacks[0];
-	vgacon_scrollback_init(0);
-}
-
-static void vgacon_scrollback_update(struct vc_data *c, int t, int count)
-{
-	void *p;
-
-	if (!vgacon_scrollback_cur->data || !vgacon_scrollback_cur->size ||
-	    c->vc_num != fg_console)
-		return;
-
-	p = (void *) (c->vc_origin + t * c->vc_size_row);
-
-	while (count--) {
-		if ((vgacon_scrollback_cur->tail + c->vc_size_row) >
-		    vgacon_scrollback_cur->size)
-			vgacon_scrollback_cur->tail = 0;
-
-		scr_memcpyw(vgacon_scrollback_cur->data +
-			    vgacon_scrollback_cur->tail,
-			    p, c->vc_size_row);
-
-		vgacon_scrollback_cur->cnt++;
-		p += c->vc_size_row;
-		vgacon_scrollback_cur->tail += c->vc_size_row;
-
-		if (vgacon_scrollback_cur->tail >= vgacon_scrollback_cur->size)
-			vgacon_scrollback_cur->tail = 0;
-
-		if (vgacon_scrollback_cur->cnt > vgacon_scrollback_cur->rows)
-			vgacon_scrollback_cur->cnt = vgacon_scrollback_cur->rows;
-
-		vgacon_scrollback_cur->cur = vgacon_scrollback_cur->cnt;
-	}
-}
-
-static void vgacon_restore_screen(struct vc_data *c)
-{
-	c->vc_origin = c->vc_visible_origin;
-	vgacon_scrollback_cur->save = 0;
-
-	if (!vga_is_gfx && !vgacon_scrollback_cur->restore) {
-		scr_memcpyw((u16 *) c->vc_origin, (u16 *) c->vc_screenbuf,
-			    c->vc_screenbuf_size > vga_vram_size ?
-			    vga_vram_size : c->vc_screenbuf_size);
-		vgacon_scrollback_cur->restore = 1;
-		vgacon_scrollback_cur->cur = vgacon_scrollback_cur->cnt;
-	}
-}
-
-static void vgacon_scrolldelta(struct vc_data *c, int lines)
-{
-	int start, end, count, soff;
-
-	if (!lines) {
-		vgacon_restore_screen(c);
-		return;
-	}
-
-	if (!vgacon_scrollback_cur->data)
-		return;
-
-	if (!vgacon_scrollback_cur->save) {
-		vgacon_cursor(c, CM_ERASE);
-		vgacon_save_screen(c);
-		c->vc_origin = (unsigned long)c->vc_screenbuf;
-		vgacon_scrollback_cur->save = 1;
-	}
-
-	vgacon_scrollback_cur->restore = 0;
-	start = vgacon_scrollback_cur->cur + lines;
-	end = start + abs(lines);
-
-	if (start < 0)
-		start = 0;
-
-	if (start > vgacon_scrollback_cur->cnt)
-		start = vgacon_scrollback_cur->cnt;
-
-	if (end < 0)
-		end = 0;
-
-	if (end > vgacon_scrollback_cur->cnt)
-		end = vgacon_scrollback_cur->cnt;
-
-	vgacon_scrollback_cur->cur = start;
-	count = end - start;
-	soff = vgacon_scrollback_cur->tail -
-		((vgacon_scrollback_cur->cnt - end) * c->vc_size_row);
-	soff -= count * c->vc_size_row;
-
-	if (soff < 0)
-		soff += vgacon_scrollback_cur->size;
-
-	count = vgacon_scrollback_cur->cnt - start;
-
-	if (count > c->vc_rows)
-		count = c->vc_rows;
-
-	if (count) {
-		int copysize;
-
-		int diff = c->vc_rows - count;
-		void *d = (void *) c->vc_visible_origin;
-		void *s = (void *) c->vc_screenbuf;
-
-		count *= c->vc_size_row;
-		/* how much memory to end of buffer left? */
-		copysize = min(count, vgacon_scrollback_cur->size - soff);
-		scr_memcpyw(d, vgacon_scrollback_cur->data + soff, copysize);
-		d += copysize;
-		count -= copysize;
-
-		if (count) {
-			scr_memcpyw(d, vgacon_scrollback_cur->data, count);
-			d += count;
-		}
-
-		if (diff)
-			scr_memcpyw(d, s, diff * c->vc_size_row);
-	} else
-		vgacon_cursor(c, CM_MOVE);
-}
-
-static void vgacon_flush_scrollback(struct vc_data *c)
-{
-	size_t size = CONFIG_VGACON_SOFT_SCROLLBACK_SIZE * 1024;
-
-	vgacon_scrollback_reset(c->vc_num, size);
-}
-#else
-#define vgacon_scrollback_startup(...) do { } while (0)
-#define vgacon_scrollback_init(...)    do { } while (0)
-#define vgacon_scrollback_update(...)  do { } while (0)
-#define vgacon_scrollback_switch(...)  do { } while (0)
-
 static void vgacon_restore_screen(struct vc_data *c)
 {
 	if (c->vc_origin != c->vc_visible_origin)
@@ -386,11 +178,6 @@ static void vgacon_scrolldelta(struct vc_data *c, int lines)
 	vga_set_mem_top(c);
 }
 
-static void vgacon_flush_scrollback(struct vc_data *c)
-{
-}
-#endif /* CONFIG_VGACON_SOFT_SCROLLBACK */
-
 static const char *vgacon_startup(void)
 {
 	const char *display_desc = NULL;
@@ -573,10 +360,7 @@ static const char *vgacon_startup(void)
 	vgacon_xres = screen_info.orig_video_cols * VGA_FONTWIDTH;
 	vgacon_yres = vga_scan_lines;
 
-	if (!vga_init_done) {
-		vgacon_scrollback_startup();
-		vga_init_done = true;
-	}
+	vga_init_done = true;
 
 	return display_desc;
 }
@@ -869,7 +653,6 @@ static int vgacon_switch(struct vc_data *c)
 			vgacon_doresize(c, c->vc_cols, c->vc_rows);
 	}
 
-	vgacon_scrollback_switch(c->vc_num);
 	return 0;		/* Redrawing not needed */
 }
 
@@ -1386,7 +1169,6 @@ static bool vgacon_scroll(struct vc_data *c, unsigned int t, unsigned int b,
 	oldo = c->vc_origin;
 	delta = lines * c->vc_size_row;
 	if (dir == SM_UP) {
-		vgacon_scrollback_update(c, t, lines);
 		if (c->vc_scr_end + delta >= vga_vram_end) {
 			scr_memcpyw((u16 *) vga_vram_base,
 				    (u16 *) (oldo + delta),
@@ -1450,7 +1232,6 @@ const struct consw vga_con = {
 	.con_save_screen = vgacon_save_screen,
 	.con_build_attr = vgacon_build_attr,
 	.con_invert_region = vgacon_invert_region,
-	.con_flush_scrollback = vgacon_flush_scrollback,
 };
 EXPORT_SYMBOL(vga_con);
 

programming_language: C