From: "Maciej W. Rozycki" <email@example.com> To: Linus Torvalds <firstname.lastname@example.org> Cc: Tetsuo Handa <email@example.com>, dri-devel <firstname.lastname@example.org>, Linux Fbdev development list <email@example.com>, Linux Kernel Mailing List <firstname.lastname@example.org>, Daniel Vetter <email@example.com>, syzbot <firstname.lastname@example.org>, Bartlomiej Zolnierkiewicz <email@example.com>, Colin King <firstname.lastname@example.org>, Greg Kroah-Hartman <email@example.com>, Jani Nikula <firstname.lastname@example.org>, Jiri Slaby <email@example.com>, syzkaller-bugs <firstname.lastname@example.org>, "Antonino A. Daplas" <email@example.com> Subject: Re: [PATCH] video: fbdev: vga16fb: fix OOB write in vga16fb_imageblit() Date: Sat, 15 May 2021 18:11:46 +0200 (CEST) [thread overview] Message-ID: <alpine.DEB.firstname.lastname@example.org> (raw) In-Reply-To: <CAHk-=wioOHwKNj8AmvXWV-oL60ae0jKswAHy9e6wCYYeA5EQXg@mail.gmail.com> On Fri, 14 May 2021, Linus Torvalds wrote: > > Overall I think it does make sense to resize the text console at any > > time, even if the visible console (VT) chosen is in the graphics mode, > > It might make sense, but only if we call the function to update the > low-level data. > > Not calling it, and then starting to randomly use the (wrong) > geometry, and just limiting it so that it's all within the buffer - > THAT does not make sense. > > So I think your patch is fundamentally wrong. It basically says "let's > use random stale incorrect data, but just make sure that the end > result is still within the allocated buffer". I guess you mean Tetsuo-san's patch, right? I haven't sent any in this discussion. > My patch is at least conceptually sane. > > An alternative would be to just remove the "vcmode != KD_GRAPHICS" > check entirely, and always call con_resize() to update the low-level > data, but honestly, that seems very likelty to break something very > fundamentally, since it's not how any of fbcon has ever been tested, Umm, there isn't much to change as far as console data structures are concerned with a resize: obviously the width and the height, which affect the size of the character/attribute buffer, and maybe some cursor data such as the size and screen coordinates. For vgacon we have: if (con_is_visible(c) && !vga_is_gfx) /* who knows */ vgacon_doresize(c, width, height); in `vgacon_resize' already, following all the sanity checks, so the CRTC isn't poked at if `vga_is_gfx', exactly as we want. I can see fbcon does not have equivalent code and instead has relied on the KD_GRAPHICS check made by the caller. Which I think has been a bug since fbcon's inception. Instead I think `fbcon_resize' ought to make all the sanity checks I can see it does and only then check for KD_GRAPHICS and if so, then exit without poking at hardware. Then upon exit from the gfx mode the `fb_set_var' call made from `fbcon_blank' will DTRT. I can try verifying the latter hypothesis, though my framebuffer setups (with DECstation hardware) have always been somewhat incomplete. I do believe I have a MIPS fbdev X server binary somewhere to fiddle with, which should work with that TGA/SFB+ video adapter I mentioned before. > Another alternative would be to just delay the resize to when vcmode > is put back to text mode again. That sounds somewhat reasonable to me, > but it's a pretty big thing. Methinks it works exactly like that already. On exit from the graphics mode (a VT switch or gfx program termination) hardware is reprogrammed according to the console geometry previously set. We just must not break it. Maciej
next prev parent reply other threads:[~2021-05-15 16:11 UTC|newest] Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top 2013-02-19 17:33 BUG: unable to handle kernel paging request at ffffc90000669000, IP: [<ffffffff8139d84a>] bitfill_un Tommi Rantala 2019-12-10 16:38 ` BUG: unable to handle kernel paging request in sys_imageblit syzbot 2020-06-19 4:56 ` syzbot 2019-12-27 7:13 ` BUG: unable to handle kernel paging request in vga16fb_imageblit syzbot 2020-05-08 7:07 ` BUG: unable to handle kernel paging request in vga16fb_imageblit (2) syzbot 2021-05-01 20:31 ` [syzbot] " syzbot 2021-05-02 1:53 ` syzbot 2021-05-03 13:41 ` Tetsuo Handa 2021-05-07 11:09 ` Tetsuo Handa 2021-05-14 16:19 ` [PATCH] video: fbdev: vga16fb: fix OOB write in vga16fb_imageblit() Tetsuo Handa 2021-05-14 17:29 ` Linus Torvalds 2021-05-14 17:37 ` Linus Torvalds 2021-05-14 18:23 ` Linus Torvalds 2021-05-14 20:25 ` Maciej W. Rozycki 2021-05-14 20:32 ` Linus Torvalds 2021-05-14 21:10 ` Linus Torvalds 2021-05-15 7:43 ` [PATCH v2] tty: vt: always invoke vc->vc_sw->con_resize callback Tetsuo Handa 2021-05-15 16:21 ` Maciej W. Rozycki 2021-05-15 16:32 ` Maciej W. Rozycki 2021-05-15 16:41 ` Linus Torvalds 2021-05-17 13:13 ` Daniel Vetter 2021-05-15 16:11 ` Maciej W. Rozycki [this message] 2021-05-17 13:07 ` [PATCH] video: fbdev: vga16fb: fix OOB write in vga16fb_imageblit() Daniel Vetter 2021-05-17 13:10 ` Daniel Vetter 2021-05-15 0:45 ` Tetsuo Handa 2020-05-12 6:55 ` BUG: unable to handle kernel paging request in bitfill_aligned syzbot 2020-10-06 8:18 ` BUG: unable to handle kernel paging request in cfb_imageblit syzbot 2020-12-18 15:26 ` syzbot 2020-12-18 15:27 ` Dmitry Vyukov
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