| Message ID | 58871f9b.d635240a.4cda6.5322@mx.google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
Hi,
Your series seems to have some coding style problems. See output below for
more information:
Type: series
Subject: [Qemu-devel] [PATCH] cirrus: fix oob access issue
Message-id: 58871f9b.d635240a.4cda6.5322@mx.google.com
=== TEST SCRIPT BEGIN ===
#!/bin/bash
BASE=base
n=1
total=$(git log --oneline $BASE.. | wc -l)
failed=0
# Useful git options
git config --local diff.renamelimit 0
git config --local diff.renames True
commits="$(git log --format=%H --reverse $BASE..)"
for c in $commits; do
echo "Checking PATCH $n/$total: $(git log -n 1 --format=%s $c)..."
if ! git show $c --format=email | ./scripts/checkpatch.pl --mailback -; then
failed=1
echo
fi
n=$((n+1))
done
exit $failed
=== TEST SCRIPT END ===
Updating 3c8cf5a9c21ff8782164d1def7f44bd888713384
From https://github.com/patchew-project/qemu
* [new tag] patchew/58871f9b.d635240a.4cda6.5322@mx.google.com -> patchew/58871f9b.d635240a.4cda6.5322@mx.google.com
Switched to a new branch 'test'
080333c cirrus: fix oob access issue
=== OUTPUT BEGIN ===
Checking PATCH 1/1: cirrus: fix oob access issue...
ERROR: spaces required around that '-' (ctx:VxV)
#21: FILE: hw/display/cirrus_vga.c:280:
+ + ((int64_t)s->cirrus_blt_height-1) * pitch
^
total: 1 errors, 0 warnings, 9 lines checked
Your patch has style problems, please review. If any of these errors
are false positives report them to the maintainer, see
CHECKPATCH in MAINTAINERS.
=== OUTPUT END ===
Test command exited with code: 1
---
Email generated automatically by Patchew [http://patchew.org/].
Please send your feedback to patchew-devel@freelists.org
On 01/24/17 10:34, Li Qiang wrote: > From: Li Qiang <liqiang6-s@360.cn> > > When doing bitblt copy in backward mode, minus the blt width first > to avoid an oob access issue. > > Signed-off-by: Li Qiang <liqiang6-s@360.cn> > --- > hw/display/cirrus_vga.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/hw/display/cirrus_vga.c b/hw/display/cirrus_vga.c > index 379910d..7ddd289 100644 > --- a/hw/display/cirrus_vga.c > +++ b/hw/display/cirrus_vga.c > @@ -277,7 +277,8 @@ static bool blit_region_is_unsafe(struct CirrusVGAState *s, > } > if (pitch < 0) { > int64_t min = addr > - + ((int64_t)s->cirrus_blt_height-1) * pitch; > + + ((int64_t)s->cirrus_blt_height-1) * pitch > + - s->cirrus_blt_width; > int32_t max = addr > + s->cirrus_blt_width; > if (min < 0 || max > s->vga.vram_size) { > I believe this is incorrect. In this case (AFAIR), "addr" points to the left-most pixel (= lowest address) of the bottom line (= highest address). This is why "max" is calculated the way it is -- to get the max address, just move to the right side of the same bottom line. Which then also means, in order to get the top left corner, you just need to subtract an integral multiple of the stride (you are already on the left side). Since the pitch is negative here, that means adding an integral multiple of the pitch. Finally, for a single-line blt, the bottom line is the only line, in which case we pitch multiplier should be (1 - 1) == 0. I think the code is correct as-is. Thanks Laszlo
> > diff --git a/hw/display/cirrus_vga.c b/hw/display/cirrus_vga.c > > index 379910d..7ddd289 100644 > > --- a/hw/display/cirrus_vga.c > > +++ b/hw/display/cirrus_vga.c > > @@ -277,7 +277,8 @@ static bool blit_region_is_unsafe(struct CirrusVGAState *s, > > } > > if (pitch < 0) { > > int64_t min = addr > > - + ((int64_t)s->cirrus_blt_height-1) * pitch; > > + + ((int64_t)s->cirrus_blt_height-1) * pitch > > + - s->cirrus_blt_width; > > int32_t max = addr > > + s->cirrus_blt_width; > > if (min < 0 || max > s->vga.vram_size) { > > > > I believe this is incorrect. In this case (AFAIR), "addr" points to the > left-most pixel (= lowest address) of the bottom line (= highest > address). If I read the code correctly it is backwards *both* x and y axis, so addr is the right-most pixel of the bottom line. cheers, Gerd
On 01/24/17 11:48, Gerd Hoffmann wrote: >>> diff --git a/hw/display/cirrus_vga.c b/hw/display/cirrus_vga.c >>> index 379910d..7ddd289 100644 >>> --- a/hw/display/cirrus_vga.c >>> +++ b/hw/display/cirrus_vga.c >>> @@ -277,7 +277,8 @@ static bool blit_region_is_unsafe(struct CirrusVGAState *s, >>> } >>> if (pitch < 0) { >>> int64_t min = addr >>> - + ((int64_t)s->cirrus_blt_height-1) * pitch; >>> + + ((int64_t)s->cirrus_blt_height-1) * pitch >>> + - s->cirrus_blt_width; >>> int32_t max = addr >>> + s->cirrus_blt_width; >>> if (min < 0 || max > s->vga.vram_size) { >>> >> >> I believe this is incorrect. In this case (AFAIR), "addr" points to the >> left-most pixel (= lowest address) of the bottom line (= highest >> address). > > If I read the code correctly it is backwards *both* x and y axis, so > addr is the right-most pixel of the bottom line. What is "max" then? If "addr" is the right-most pixel of the bottom line, then "max" has the highest address just past the rectangle, and then adding anything non-negative to it makes no sense. Adding a negative value to it (i.e., if s->cirrus_blt_width were negative) might make sense, for getting the leftmost pixel on the bottom line, but then that "max" value shouldn't be used for the final boundary check (against RAM size). ... Really as I remember it from the downstream review, the pitch is negative (bottom-up), but the horizontal direction remains left to right. 0-----------------------------------------------------------------> x | | addr + (height - 1) * pitch, pitch < 0, height > 0 | | | v | *--------------------* <-- addr + (height - 1) * pitch + width | | | | | | | | | | | | | | | | | | | *--------------------* | ^ ^ | | | | addr max = addr + width, width > 0 | v y The rightmost column represented here is exclusive, the rest is inclusive. Thanks Laszlo
On 01/24/17 12:17, Laszlo Ersek wrote: > On 01/24/17 11:48, Gerd Hoffmann wrote: >>>> diff --git a/hw/display/cirrus_vga.c b/hw/display/cirrus_vga.c >>>> index 379910d..7ddd289 100644 >>>> --- a/hw/display/cirrus_vga.c >>>> +++ b/hw/display/cirrus_vga.c >>>> @@ -277,7 +277,8 @@ static bool blit_region_is_unsafe(struct CirrusVGAState *s, >>>> } >>>> if (pitch < 0) { >>>> int64_t min = addr >>>> - + ((int64_t)s->cirrus_blt_height-1) * pitch; >>>> + + ((int64_t)s->cirrus_blt_height-1) * pitch >>>> + - s->cirrus_blt_width; >>>> int32_t max = addr >>>> + s->cirrus_blt_width; >>>> if (min < 0 || max > s->vga.vram_size) { >>>> >>> >>> I believe this is incorrect. In this case (AFAIR), "addr" points to the >>> left-most pixel (= lowest address) of the bottom line (= highest >>> address). >> >> If I read the code correctly it is backwards *both* x and y axis, so >> addr is the right-most pixel of the bottom line. > > What is "max" then? If "addr" is the right-most pixel of the bottom > line, then "max" has the highest address just past the rectangle, and > then adding anything non-negative to it makes no sense. sorry, over-editing caused a bug. I meant If "addr" is the right-most pixel of the bottom line (or just one past that), then *it* has the highest address, and then adding anything non-negative to it (i.e., how we end up with "max") makes no sense. I should have started with the diagram, not with the text. Thanks Laszlo > > Adding a negative value to it (i.e., if s->cirrus_blt_width were > negative) might make sense, for getting the leftmost pixel on the bottom > line, but then that "max" value shouldn't be used for the final boundary > check (against RAM size). > > ... Really as I remember it from the downstream review, the pitch is > negative (bottom-up), but the horizontal direction remains left to right. > > 0-----------------------------------------------------------------> x > | > | addr + (height - 1) * pitch, pitch < 0, height > 0 > | | > | v > | *--------------------* <-- addr + (height - 1) * pitch + width > | | | > | | | > | | | > | | | > | | | > | | | > | *--------------------* > | ^ ^ > | | | > | addr max = addr + width, width > 0 > | > v > y > > > The rightmost column represented here is exclusive, the rest is inclusive. > > Thanks > Laszlo >
> >>> if (pitch < 0) { > >>> int64_t min = addr > >>> - + ((int64_t)s->cirrus_blt_height-1) * pitch; > >>> + + ((int64_t)s->cirrus_blt_height-1) * pitch > >>> + - s->cirrus_blt_width; > >>> int32_t max = addr > >>> + s->cirrus_blt_width; > >>> if (min < 0 || max > s->vga.vram_size) { > >>> > >> > >> I believe this is incorrect. In this case (AFAIR), "addr" points to the > >> left-most pixel (= lowest address) of the bottom line (= highest > >> address). > > > > If I read the code correctly it is backwards *both* x and y axis, so > > addr is the right-most pixel of the bottom line. > > What is "max" then? If "addr" is the right-most pixel of the bottom > line, then "max" has the highest address just past the rectangle, and > then adding anything non-negative to it makes no sense. That is (with the patch applied) inconsistent indeed. We must either subtract s->cirrus_blt_width from min (addr == right-most), or add it to max (addr == left-most), but certainly not both. > ... Really as I remember it from the downstream review, the pitch is > negative (bottom-up), but the horizontal direction remains left to right. Looking at cirrus_vga_rop.h I see: - cirrus_bitblt_rop_fwd_*() increment src and dst while walking the scanline, and - cirrus_bitblt_rop_bkwd_*() decrement src and dst ... I still think x axis goes backwards too and therefore addr is the right-most pixel. cheers, Gerd
On Tue, Jan 24, 2017 at 01:29:58PM +0100, Gerd Hoffmann wrote: > > >>> if (pitch < 0) { > > >>> int64_t min = addr > > >>> - + ((int64_t)s->cirrus_blt_height-1) * pitch; > > >>> + + ((int64_t)s->cirrus_blt_height-1) * pitch > > >>> + - s->cirrus_blt_width; > > >>> int32_t max = addr > > >>> + s->cirrus_blt_width; > > >>> if (min < 0 || max > s->vga.vram_size) { > > >>> > > >> > > >> I believe this is incorrect. In this case (AFAIR), "addr" points to the > > >> left-most pixel (= lowest address) of the bottom line (= highest > > >> address). > > > > > > If I read the code correctly it is backwards *both* x and y axis, so > > > addr is the right-most pixel of the bottom line. > > > > What is "max" then? If "addr" is the right-most pixel of the bottom > > line, then "max" has the highest address just past the rectangle, and > > then adding anything non-negative to it makes no sense. > > That is (with the patch applied) inconsistent indeed. We must either > subtract s->cirrus_blt_width from min (addr == right-most), or add it to > max (addr == left-most), but certainly not both. > > > ... Really as I remember it from the downstream review, the pitch is > > negative (bottom-up), but the horizontal direction remains left to right. > > Looking at cirrus_vga_rop.h I see: > - cirrus_bitblt_rop_fwd_*() increment src and dst while walking the > scanline, and > - cirrus_bitblt_rop_bkwd_*() decrement src and dst ... > > I still think x axis goes backwards too and therefore addr is the > right-most pixel. I agree. Seeing how I've already been reading through that code I thought I'd go over it again and too would say both min and max need to be adapted: if (pitch < 0) { int64_t min = addr + ((int64_t)s->cirrus_blt_height-1) * pitch; + - s->cirrus_blt_width; int32_t max = addr - + s->cirrus_blt_width; if (min < 0 || max > s->vga.vram_size) { Here's the rest of my analysis in case anyone's interested (mostly to justify the first part of this mail): -) Sign of the blit width/height & pitch values at the beginning: | s->cirrus_blt_width = (s->vga.gr[0x20] | (s->vga.gr[0x21] << 8)) + 1; | s->cirrus_blt_height = (s->vga.gr[0x22] | (s->vga.gr[0x23] << 8)) + 1; | s->cirrus_blt_dstpitch = (s->vga.gr[0x24] | (s->vga.gr[0x25] << 8)); | s->cirrus_blt_srcpitch = (s->vga.gr[0x26] | (s->vga.gr[0x27] << 8)); vga.gr is an uint8_t[] ==> all values are positive at this point -) Backward blits invert the sign: | if (s->cirrus_blt_mode & CIRRUS_BLTMODE_BACKWARDS) { | s->cirrus_blt_dstpitch = -s->cirrus_blt_dstpitch; | s->cirrus_blt_srcpitch = -s->cirrus_blt_srcpitch; Starting with the simple one: -) Forward blits from cirrus_vga_rop.h: Width (which is positive) is subtracted from the pitch (which is also positive), turning srcpitch and dstpitch into values representing the remaining bytes after the current row. The pattern below repeats for all functions: |static void |glue(cirrus_bitblt_rop_fwd_, ROP_NAME)(CirrusVGAState *s, | uint8_t *dst,const uint8_t *src, | int dstpitch,int srcpitch, | int bltwidth,int bltheight) |{ | int x,y; | dstpitch -= bltwidth; | srcpitch -= bltwidth; | | if (bltheight > 1 && (dstpitch < 0 || srcpitch < 0)) { | return; | } | | for (y = 0; y < bltheight; y++) { | for (x = 0; x < bltwidth; x++) { | ROP_OP(dst, *src); | dst++; | src++; | } | dst += dstpitch; | src += srcpitch; | } |} The first access to src/dst is unmodified, so the lowest accessed address is the initial address. Some functions iterate through pairs: | for (x = 0; x < bltwidth; x+=2) { | p1 = *dst; | p2 = *(dst+1); Since the loop uses `x += 2` this `+1` should not go out of bounds provided the width is even (which if not the case should at least have an even pitch value). Conclusion for forward blits: We use (start + pitch * (height-1) + width) which seems obvious for the main pattern but we also need to assert that the 2nd example above cannot access an additional byte with *(dst+1) after this value. This seems to be okay: for an odd width eg. 5, the highest x value the loop body is executed with will be 4, and we access 4+1=5 at most. -) Backward blits form cirrus_vga_rop.h: (Pitch is negative, width is positive.) They "add" the width to the pitch (essentially reducing its length), turning the 'dstpitch' and 'srcpitch' variables - as with forward blits - into values representing only the *remaining* bytes "in front of" the data in the current row. The pattern: |glue(cirrus_bitblt_rop_bkwd_, ROP_NAME)(CirrusVGAState *s, | uint8_t *dst,const uint8_t *src, | int dstpitch,int srcpitch, | int bltwidth,int bltheight) |{ | int x,y; | dstpitch += bltwidth; | srcpitch += bltwidth; | for (y = 0; y < bltheight; y++) { | for (x = 0; x < bltwidth; x++) { | ROP_OP(dst, *src); | dst--; | src--; | } | dst += dstpitch; | src += srcpitch; | } |} Pitch is negative, first value touched is 'dst' and 'src' unmodified. The same pattern is used throughout the rest of the header. As far as I can see the 'bkwd' ops only ever subtract from the addresses (note that in `X += Xpitch`, Xpitch is negative), and like with forward blits we have at most a 'src-1' (not +1 this time) access in a loop body where we iterate over pairs, so the same reasoning holds here. Conclusion for backward blits: 1) We should use: (addr + (height-1)*pitch - width) but originally missed the `-with` part there. 2) The maximum value should actually be `addr` itself as far as I can tell, so the `+ s->cirrus_blt_width` for max should be dropped. The functions in cirrus_vga_rop2.h are a bitmore complex. The safety checks *may* even be too strict in some cases but I haven't seen any aftefacts coming from too strong *range* checks (only the zero pitch check for which I sent a patch yesterday which I need to make a v2 for now :|). (Too strict in the pattern functions which clamp the source offsets with bitands, but I really don't feel like looking deep enough into this to make the checks even lighter :p)
On 01/24/17 16:31, Wolfgang Bumiller wrote: > On Tue, Jan 24, 2017 at 01:29:58PM +0100, Gerd Hoffmann wrote: >>>>>> if (pitch < 0) { >>>>>> int64_t min = addr >>>>>> - + ((int64_t)s->cirrus_blt_height-1) * pitch; >>>>>> + + ((int64_t)s->cirrus_blt_height-1) * pitch >>>>>> + - s->cirrus_blt_width; >>>>>> int32_t max = addr >>>>>> + s->cirrus_blt_width; >>>>>> if (min < 0 || max > s->vga.vram_size) { >>>>>> >>>>> >>>>> I believe this is incorrect. In this case (AFAIR), "addr" points to the >>>>> left-most pixel (= lowest address) of the bottom line (= highest >>>>> address). >>>> >>>> If I read the code correctly it is backwards *both* x and y axis, so >>>> addr is the right-most pixel of the bottom line. >>> >>> What is "max" then? If "addr" is the right-most pixel of the bottom >>> line, then "max" has the highest address just past the rectangle, and >>> then adding anything non-negative to it makes no sense. >> >> That is (with the patch applied) inconsistent indeed. We must either >> subtract s->cirrus_blt_width from min (addr == right-most), or add it to >> max (addr == left-most), but certainly not both. >> >>> ... Really as I remember it from the downstream review, the pitch is >>> negative (bottom-up), but the horizontal direction remains left to right. >> >> Looking at cirrus_vga_rop.h I see: >> - cirrus_bitblt_rop_fwd_*() increment src and dst while walking the >> scanline, and >> - cirrus_bitblt_rop_bkwd_*() decrement src and dst ... >> >> I still think x axis goes backwards too and therefore addr is the >> right-most pixel. > > I agree. > > Seeing how I've already been reading through that code I thought I'd > go over it again and too would say both min and max need to be adapted: > > if (pitch < 0) { > int64_t min = addr > + ((int64_t)s->cirrus_blt_height-1) * pitch; > + - s->cirrus_blt_width; > int32_t max = addr > - + s->cirrus_blt_width; > if (min < 0 || max > s->vga.vram_size) { > > > Here's the rest of my analysis in case anyone's interested (mostly to > justify the first part of this mail): > > -) Sign of the blit width/height & pitch values at the beginning: > > | s->cirrus_blt_width = (s->vga.gr[0x20] | (s->vga.gr[0x21] << 8)) + 1; > | s->cirrus_blt_height = (s->vga.gr[0x22] | (s->vga.gr[0x23] << 8)) + 1; > | s->cirrus_blt_dstpitch = (s->vga.gr[0x24] | (s->vga.gr[0x25] << 8)); > | s->cirrus_blt_srcpitch = (s->vga.gr[0x26] | (s->vga.gr[0x27] << 8)); > > vga.gr is an uint8_t[] > > ==> all values are positive at this point > > -) Backward blits invert the sign: > > | if (s->cirrus_blt_mode & CIRRUS_BLTMODE_BACKWARDS) { > | s->cirrus_blt_dstpitch = -s->cirrus_blt_dstpitch; > | s->cirrus_blt_srcpitch = -s->cirrus_blt_srcpitch; > > > Starting with the simple one: > > -) Forward blits from cirrus_vga_rop.h: > Width (which is positive) is subtracted from the pitch (which > is also positive), turning srcpitch and dstpitch into values > representing the remaining bytes after the current row. > The pattern below repeats for all functions: > > |static void > |glue(cirrus_bitblt_rop_fwd_, ROP_NAME)(CirrusVGAState *s, > | uint8_t *dst,const uint8_t *src, > | int dstpitch,int srcpitch, > | int bltwidth,int bltheight) > |{ > | int x,y; > | dstpitch -= bltwidth; > | srcpitch -= bltwidth; > | > | if (bltheight > 1 && (dstpitch < 0 || srcpitch < 0)) { > | return; > | } > | > | for (y = 0; y < bltheight; y++) { > | for (x = 0; x < bltwidth; x++) { > | ROP_OP(dst, *src); > | dst++; > | src++; > | } > | dst += dstpitch; > | src += srcpitch; > | } > |} > > The first access to src/dst is unmodified, so the lowest accessed > address is the initial address. > > Some functions iterate through pairs: > | for (x = 0; x < bltwidth; x+=2) { > | p1 = *dst; > | p2 = *(dst+1); > Since the loop uses `x += 2` this `+1` should not go out of bounds > provided the width is even (which if not the case should at least > have an even pitch value). > > Conclusion for forward blits: > We use (start + pitch * (height-1) + width) which seems obvious for > the main pattern but we also need to assert that the 2nd example > above cannot access an additional byte with *(dst+1) after this > value. This seems to be okay: for an odd width eg. 5, the highest > x value the loop body is executed with will be 4, and we access > 4+1=5 at most. > > -) Backward blits form cirrus_vga_rop.h: > (Pitch is negative, width is positive.) > They "add" the width to the pitch (essentially reducing its length), > turning the 'dstpitch' and 'srcpitch' variables - as with forward > blits - into values representing only the *remaining* bytes "in > front of" the data in the current row. > The pattern: > > |glue(cirrus_bitblt_rop_bkwd_, ROP_NAME)(CirrusVGAState *s, > | uint8_t *dst,const uint8_t *src, > | int dstpitch,int srcpitch, > | int bltwidth,int bltheight) > |{ > | int x,y; > | dstpitch += bltwidth; > | srcpitch += bltwidth; > | for (y = 0; y < bltheight; y++) { > | for (x = 0; x < bltwidth; x++) { > | ROP_OP(dst, *src); > | dst--; > | src--; > | } > | dst += dstpitch; > | src += srcpitch; > | } > |} > > Pitch is negative, first value touched is 'dst' and 'src' > unmodified. The same pattern is used throughout the rest of the > header. > > As far as I can see the 'bkwd' ops only ever subtract from the > addresses (note that in `X += Xpitch`, Xpitch is negative), and > like with forward blits we have at most a 'src-1' (not +1 this > time) access in a loop body where we iterate over pairs, so the > same reasoning holds here. > > Conclusion for backward blits: > 1) We should use: (addr + (height-1)*pitch - width) but originally > missed the `-with` part there. > > 2) The maximum value should actually be `addr` itself as far as I > can tell, so the `+ s->cirrus_blt_width` for max should be > dropped. You (and Gerd) are correct. I've just looked up my original downstream review of this patch ("cirrus: fix blit region check", now commit d3532a0db022 in upstream). While at that time I range-checked everything and their grandma, and found (with Gerd's feedback) those aspects safe, I only *assumed* that for the negative pitch case, "addr" would point to the bottom left corner of the rectangle: On 01/15/15 12:21, Laszlo Ersek wrote: > The negative pitch means (I think) that "addr" points to the lower > left corner of the rectangle. > > The second part guarantees that the last blitted byte fits (lower > right corner). To which Gerd responded "upper left". In retrospect I don't understand why we didn't discuss that question further, as it now seems that we were both wrong -- "addr" stands for bottom right, in the negative pitch case. So, I agree that we should subtract width from both min and max here. The current discussion has proved that the blit_region_is_unsafe() function is incorrect ATM -- in a backwards blit, the guest can advance to lower addresses than the "min" value we calculate and enforce to be non-negative. Unfortunately, the original patch was meant to address the then-embargoed CVE-2014-8106. Since we have a bug in that code (= a security fix), this issue should have been reported privately as well, and another embargo would have been justified. We need to fix this ASAP, and a new CVE should be requested; the cat is now out of the bag. Thanks Laszlo > > > The functions in cirrus_vga_rop2.h are a bitmore complex. The safety > checks *may* even be too strict in some cases but I haven't seen any > aftefacts coming from too strong *range* checks (only the zero pitch > check for which I sent a patch yesterday which I need to make a v2 for > now :|). > (Too strict in the pattern functions which clamp the source offsets > with bitands, but I really don't feel like looking deep enough into > this to make the checks even lighter :p) >
2017-01-25 0:12 GMT+08:00 Laszlo Ersek <lersek@redhat.com>: > On 01/24/17 16:31, Wolfgang Bumiller wrote: > > On Tue, Jan 24, 2017 at 01:29:58PM +0100, Gerd Hoffmann wrote: > >>>>>> if (pitch < 0) { > >>>>>> int64_t min = addr > >>>>>> - + ((int64_t)s->cirrus_blt_height-1) * pitch; > >>>>>> + + ((int64_t)s->cirrus_blt_height-1) * pitch > >>>>>> + - s->cirrus_blt_width; > >>>>>> int32_t max = addr > >>>>>> + s->cirrus_blt_width; > >>>>>> if (min < 0 || max > s->vga.vram_size) { > >>>>>> > >>>>> > >>>>> I believe this is incorrect. In this case (AFAIR), "addr" points to > the > >>>>> left-most pixel (= lowest address) of the bottom line (= highest > >>>>> address). > >>>> > >>>> If I read the code correctly it is backwards *both* x and y axis, so > >>>> addr is the right-most pixel of the bottom line. > >>> > >>> What is "max" then? If "addr" is the right-most pixel of the bottom > >>> line, then "max" has the highest address just past the rectangle, and > >>> then adding anything non-negative to it makes no sense. > >> > >> That is (with the patch applied) inconsistent indeed. We must either > >> subtract s->cirrus_blt_width from min (addr == right-most), or add it to > >> max (addr == left-most), but certainly not both. > >> > >>> ... Really as I remember it from the downstream review, the pitch is > >>> negative (bottom-up), but the horizontal direction remains left to > right. > >> > >> Looking at cirrus_vga_rop.h I see: > >> - cirrus_bitblt_rop_fwd_*() increment src and dst while walking the > >> scanline, and > >> - cirrus_bitblt_rop_bkwd_*() decrement src and dst ... > >> > >> I still think x axis goes backwards too and therefore addr is the > >> right-most pixel. > > > > I agree. > > > > Seeing how I've already been reading through that code I thought I'd > > go over it again and too would say both min and max need to be adapted: > > > > if (pitch < 0) { > > int64_t min = addr > > + ((int64_t)s->cirrus_blt_height-1) * pitch; > > + - s->cirrus_blt_width; > > int32_t max = addr > > - + s->cirrus_blt_width; > > if (min < 0 || max > s->vga.vram_size) { > > > > > > Here's the rest of my analysis in case anyone's interested (mostly to > > justify the first part of this mail): > > > > -) Sign of the blit width/height & pitch values at the beginning: > > > > | s->cirrus_blt_width = (s->vga.gr[0x20] | (s->vga.gr[0x21] << > 8)) + 1; > > | s->cirrus_blt_height = (s->vga.gr[0x22] | (s->vga.gr[0x23] << > 8)) + 1; > > | s->cirrus_blt_dstpitch = (s->vga.gr[0x24] | (s->vga.gr[0x25] > << 8)); > > | s->cirrus_blt_srcpitch = (s->vga.gr[0x26] | (s->vga.gr[0x27] > << 8)); > > > > vga.gr is an uint8_t[] > > > > ==> all values are positive at this point > > > > -) Backward blits invert the sign: > > > > | if (s->cirrus_blt_mode & CIRRUS_BLTMODE_BACKWARDS) { > > | s->cirrus_blt_dstpitch = -s->cirrus_blt_dstpitch; > > | s->cirrus_blt_srcpitch = -s->cirrus_blt_srcpitch; > > > > > > Starting with the simple one: > > > > -) Forward blits from cirrus_vga_rop.h: > > Width (which is positive) is subtracted from the pitch (which > > is also positive), turning srcpitch and dstpitch into values > > representing the remaining bytes after the current row. > > The pattern below repeats for all functions: > > > > |static void > > |glue(cirrus_bitblt_rop_fwd_, ROP_NAME)(CirrusVGAState *s, > > | uint8_t *dst,const uint8_t *src, > > | int dstpitch,int srcpitch, > > | int bltwidth,int bltheight) > > |{ > > | int x,y; > > | dstpitch -= bltwidth; > > | srcpitch -= bltwidth; > > | > > | if (bltheight > 1 && (dstpitch < 0 || srcpitch < 0)) { > > | return; > > | } > > | > > | for (y = 0; y < bltheight; y++) { > > | for (x = 0; x < bltwidth; x++) { > > | ROP_OP(dst, *src); > > | dst++; > > | src++; > > | } > > | dst += dstpitch; > > | src += srcpitch; > > | } > > |} > > > > The first access to src/dst is unmodified, so the lowest accessed > > address is the initial address. > > > > Some functions iterate through pairs: > > | for (x = 0; x < bltwidth; x+=2) { > > | p1 = *dst; > > | p2 = *(dst+1); > > Since the loop uses `x += 2` this `+1` should not go out of bounds > > provided the width is even (which if not the case should at least > > have an even pitch value). > > > > Conclusion for forward blits: > > We use (start + pitch * (height-1) + width) which seems obvious for > > the main pattern but we also need to assert that the 2nd example > > above cannot access an additional byte with *(dst+1) after this > > value. This seems to be okay: for an odd width eg. 5, the highest > > x value the loop body is executed with will be 4, and we access > > 4+1=5 at most. > > > > -) Backward blits form cirrus_vga_rop.h: > > (Pitch is negative, width is positive.) > > They "add" the width to the pitch (essentially reducing its length), > > turning the 'dstpitch' and 'srcpitch' variables - as with forward > > blits - into values representing only the *remaining* bytes "in > > front of" the data in the current row. > > The pattern: > > > > |glue(cirrus_bitblt_rop_bkwd_, ROP_NAME)(CirrusVGAState *s, > > | uint8_t *dst,const > uint8_t *src, > > | int dstpitch,int srcpitch, > > | int bltwidth,int > bltheight) > > |{ > > | int x,y; > > | dstpitch += bltwidth; > > | srcpitch += bltwidth; > > | for (y = 0; y < bltheight; y++) { > > | for (x = 0; x < bltwidth; x++) { > > | ROP_OP(dst, *src); > > | dst--; > > | src--; > > | } > > | dst += dstpitch; > > | src += srcpitch; > > | } > > |} > > > > Pitch is negative, first value touched is 'dst' and 'src' > > unmodified. The same pattern is used throughout the rest of the > > header. > > > > As far as I can see the 'bkwd' ops only ever subtract from the > > addresses (note that in `X += Xpitch`, Xpitch is negative), and > > like with forward blits we have at most a 'src-1' (not +1 this > > time) access in a loop body where we iterate over pairs, so the > > same reasoning holds here. > > > > Conclusion for backward blits: > > 1) We should use: (addr + (height-1)*pitch - width) but originally > > missed the `-with` part there. > > > > 2) The maximum value should actually be `addr` itself as far as I > > can tell, so the `+ s->cirrus_blt_width` for max should be > > dropped. > > You (and Gerd) are correct. > > I've just looked up my original downstream review of this patch > ("cirrus: fix blit region check", now commit d3532a0db022 in upstream). > While at that time I range-checked everything and their grandma, and > found (with Gerd's feedback) those aspects safe, I only *assumed* that > for the negative pitch case, "addr" would point to the bottom left > corner of the rectangle: > > On 01/15/15 12:21, Laszlo Ersek wrote: > > > The negative pitch means (I think) that "addr" points to the lower > > left corner of the rectangle. > > > > The second part guarantees that the last blitted byte fits (lower > > right corner). > > To which Gerd responded "upper left". In retrospect I don't understand > why we didn't discuss that question further, as it now seems that we > were both wrong -- "addr" stands for bottom right, in the negative pitch > case. > > So, I agree that we should subtract width from both min and max here. > > I have read all the discuss, very long and useful, but I think I still need some time to get a full understand. So I think one of you can provide the formal patch to describe the issue in more detail. > The current discussion has proved that the blit_region_is_unsafe() > function is incorrect ATM -- in a backwards blit, the guest can advance > to lower addresses than the "min" value we calculate and enforce to be > non-negative. > > Unfortunately, the original patch was meant to address the > then-embargoed CVE-2014-8106. Since we have a bug in that code (= a > security fix), this issue should have been reported privately as well, > and another embargo would have been justified. We need to fix this ASAP, > and a new CVE should be requested; the cat is now out of the bag. > > Actually, I have reported this issue several months ago to redhat. But it seems this is not get the focus, so I tried myself to address it. As I'm not a vga specialist I think this issue is a little difficult. Thanks! Li Qiang / Cloud Security Team, Qihoo 360 Inc > Thanks > Laszlo > > > > > > > The functions in cirrus_vga_rop2.h are a bitmore complex. The safety > > checks *may* even be too strict in some cases but I haven't seen any > > aftefacts coming from too strong *range* checks (only the zero pitch > > check for which I sent a patch yesterday which I need to make a v2 for > > now :|). > > (Too strict in the pattern functions which clamp the source offsets > > with bitands, but I really don't feel like looking deep enough into > > this to make the checks even lighter :p) > > > >
On 01/25/17 02:18, Li Qiang wrote: > > > 2017-01-25 0:12 GMT+08:00 Laszlo Ersek <lersek@redhat.com > <mailto:lersek@redhat.com>>: > > On 01/24/17 16:31, Wolfgang Bumiller wrote: > > On Tue, Jan 24, 2017 at 01:29:58PM +0100, Gerd Hoffmann wrote: > >>>>>> if (pitch < 0) { > >>>>>> int64_t min = addr > >>>>>> - + ((int64_t)s->cirrus_blt_height-1) * pitch; > >>>>>> + + ((int64_t)s->cirrus_blt_height-1) * pitch > >>>>>> + - s->cirrus_blt_width; > >>>>>> int32_t max = addr > >>>>>> + s->cirrus_blt_width; > >>>>>> if (min < 0 || max > s->vga.vram_size) { > >>>>>> > >>>>> > >>>>> I believe this is incorrect. In this case (AFAIR), "addr" > points to the > >>>>> left-most pixel (= lowest address) of the bottom line (= highest > >>>>> address). > >>>> > >>>> If I read the code correctly it is backwards *both* x and y > axis, so > >>>> addr is the right-most pixel of the bottom line. > >>> > >>> What is "max" then? If "addr" is the right-most pixel of the bottom > >>> line, then "max" has the highest address just past the > rectangle, and > >>> then adding anything non-negative to it makes no sense. > >> > >> That is (with the patch applied) inconsistent indeed. We must either > >> subtract s->cirrus_blt_width from min (addr == right-most), or > add it to > >> max (addr == left-most), but certainly not both. > >> > >>> ... Really as I remember it from the downstream review, the pitch is > >>> negative (bottom-up), but the horizontal direction remains left > to right. > >> > >> Looking at cirrus_vga_rop.h I see: > >> - cirrus_bitblt_rop_fwd_*() increment src and dst while walking the > >> scanline, and > >> - cirrus_bitblt_rop_bkwd_*() decrement src and dst ... > >> > >> I still think x axis goes backwards too and therefore addr is the > >> right-most pixel. > > > > I agree. > > > > Seeing how I've already been reading through that code I thought I'd > > go over it again and too would say both min and max need to be > adapted: > > > > if (pitch < 0) { > > int64_t min = addr > > + ((int64_t)s->cirrus_blt_height-1) * pitch; > > + - s->cirrus_blt_width; > > int32_t max = addr > > - + s->cirrus_blt_width; > > if (min < 0 || max > s->vga.vram_size) { > > > > > > Here's the rest of my analysis in case anyone's interested (mostly to > > justify the first part of this mail): > > > > -) Sign of the blit width/height & pitch values at the beginning: > > > > | s->cirrus_blt_width = (s->vga.gr <http://vga.gr>[0x20] | > (s->vga.gr <http://vga.gr>[0x21] << 8)) + 1; > > | s->cirrus_blt_height = (s->vga.gr <http://vga.gr>[0x22] | > (s->vga.gr <http://vga.gr>[0x23] << 8)) + 1; > > | s->cirrus_blt_dstpitch = (s->vga.gr <http://vga.gr>[0x24] > | (s->vga.gr <http://vga.gr>[0x25] << 8)); > > | s->cirrus_blt_srcpitch = (s->vga.gr <http://vga.gr>[0x26] > | (s->vga.gr <http://vga.gr>[0x27] << 8)); > > > > vga.gr <http://vga.gr> is an uint8_t[] > > > > ==> all values are positive at this point > > > > -) Backward blits invert the sign: > > > > | if (s->cirrus_blt_mode & CIRRUS_BLTMODE_BACKWARDS) { > > | s->cirrus_blt_dstpitch = -s->cirrus_blt_dstpitch; > > | s->cirrus_blt_srcpitch = -s->cirrus_blt_srcpitch; > > > > > > Starting with the simple one: > > > > -) Forward blits from cirrus_vga_rop.h: > > Width (which is positive) is subtracted from the pitch (which > > is also positive), turning srcpitch and dstpitch into values > > representing the remaining bytes after the current row. > > The pattern below repeats for all functions: > > > > |static void > > |glue(cirrus_bitblt_rop_fwd_, ROP_NAME)(CirrusVGAState *s, > > | uint8_t *dst,const uint8_t *src, > > | int dstpitch,int srcpitch, > > | int bltwidth,int bltheight) > > |{ > > | int x,y; > > | dstpitch -= bltwidth; > > | srcpitch -= bltwidth; > > | > > | if (bltheight > 1 && (dstpitch < 0 || srcpitch < 0)) { > > | return; > > | } > > | > > | for (y = 0; y < bltheight; y++) { > > | for (x = 0; x < bltwidth; x++) { > > | ROP_OP(dst, *src); > > | dst++; > > | src++; > > | } > > | dst += dstpitch; > > | src += srcpitch; > > | } > > |} > > > > The first access to src/dst is unmodified, so the lowest accessed > > address is the initial address. > > > > Some functions iterate through pairs: > > | for (x = 0; x < bltwidth; x+=2) { > > | p1 = *dst; > > | p2 = *(dst+1); > > Since the loop uses `x += 2` this `+1` should not go out of bounds > > provided the width is even (which if not the case should at least > > have an even pitch value). > > > > Conclusion for forward blits: > > We use (start + pitch * (height-1) + width) which seems obvious for > > the main pattern but we also need to assert that the 2nd example > > above cannot access an additional byte with *(dst+1) after this > > value. This seems to be okay: for an odd width eg. 5, the highest > > x value the loop body is executed with will be 4, and we access > > 4+1=5 at most. > > > > -) Backward blits form cirrus_vga_rop.h: > > (Pitch is negative, width is positive.) > > They "add" the width to the pitch (essentially reducing its > length), > > turning the 'dstpitch' and 'srcpitch' variables - as with forward > > blits - into values representing only the *remaining* bytes "in > > front of" the data in the current row. > > The pattern: > > > > |glue(cirrus_bitblt_rop_bkwd_, ROP_NAME)(CirrusVGAState *s, > > | uint8_t *dst,const > uint8_t *src, > > | int dstpitch,int > srcpitch, > > | int bltwidth,int > bltheight) > > |{ > > | int x,y; > > | dstpitch += bltwidth; > > | srcpitch += bltwidth; > > | for (y = 0; y < bltheight; y++) { > > | for (x = 0; x < bltwidth; x++) { > > | ROP_OP(dst, *src); > > | dst--; > > | src--; > > | } > > | dst += dstpitch; > > | src += srcpitch; > > | } > > |} > > > > Pitch is negative, first value touched is 'dst' and 'src' > > unmodified. The same pattern is used throughout the rest of the > > header. > > > > As far as I can see the 'bkwd' ops only ever subtract from the > > addresses (note that in `X += Xpitch`, Xpitch is negative), and > > like with forward blits we have at most a 'src-1' (not +1 this > > time) access in a loop body where we iterate over pairs, so the > > same reasoning holds here. > > > > Conclusion for backward blits: > > 1) We should use: (addr + (height-1)*pitch - width) but originally > > missed the `-with` part there. > > > > 2) The maximum value should actually be `addr` itself as far as I > > can tell, so the `+ s->cirrus_blt_width` for max should be > > dropped. > > You (and Gerd) are correct. > > I've just looked up my original downstream review of this patch > ("cirrus: fix blit region check", now commit d3532a0db022 in upstream). > While at that time I range-checked everything and their grandma, and > found (with Gerd's feedback) those aspects safe, I only *assumed* that > for the negative pitch case, "addr" would point to the bottom left > corner of the rectangle: > > On 01/15/15 12:21, Laszlo Ersek wrote: > > > The negative pitch means (I think) that "addr" points to the lower > > left corner of the rectangle. > > > > The second part guarantees that the last blitted byte fits (lower > > right corner). > > To which Gerd responded "upper left". In retrospect I don't understand > why we didn't discuss that question further, as it now seems that we > were both wrong -- "addr" stands for bottom right, in the negative pitch > case. > > So, I agree that we should subtract width from both min and max here. > > > I have read all the discuss, very long and useful, but I think I still > need some > time to get a full understand. So I think one of you can provide the > formal patch to > describe the issue in more detail. Your patch is almost correct. The blit_region_is_unsafe() function determines, in advance, the address range that a blit operation intends to access (read or overwrite), as configured by the guest. If the address range is not a subset of the VGA RAM (i.e., the guest tries to access QEMU memory before the VGA RAM, or after it), then the operation is rejected. There are forward and backward advancing variants of the blit operations. They translate to positive and negative pitches, respectively. On the negative pitch branch, the address range that blit_region_is_unsafe() calculates and verifies against the VGA RAM is incorrect; it does not actually match the address range that the actual, later execution of the blit operation will scan over. Therefore it is possible for the guest to construct a blit request that passes the (incorrect) check, but then the actual address range scan gets outside of the restricted area. The issue is that the starting address for the backwards advancing blit ops, as interpreted by the actual address range scan, is the bottom right corner of the rectangle. But the range check in blit_region_is_unsafe() considers the starting address to identify the bottom left corner of the rectangle. Your change corrects the interpretation of "addr" in blit_region_is_unsafe() for "min", which stands for "top left", but in parallel, the "max" value has to be corrected too; otherwise "max" will be too large (it will point past the bottom right corner of the rectangle). > > > > The current discussion has proved that the blit_region_is_unsafe() > function is incorrect ATM -- in a backwards blit, the guest can advance > to lower addresses than the "min" value we calculate and enforce to be > non-negative. > > Unfortunately, the original patch was meant to address the > then-embargoed CVE-2014-8106. Since we have a bug in that code (= a > security fix), this issue should have been reported privately as well, > and another embargo would have been justified. We need to fix this ASAP, > and a new CVE should be requested; the cat is now out of the bag. > > > Actually, I have reported this issue several months ago to redhat. But > it seems > this is not get the focus, That's a cold shower. :( > so I tried myself to address it. As I'm not a > vga specialist I think > this issue is a little difficult. It is. Gerd, do you think you can raise this with the SRT? (I mean, if you agree that this is a security issue.) Thanks Laszlo > > Thanks! > > Li Qiang / Cloud Security Team, Qihoo 360 Inc > > > Thanks > Laszlo > > > > > > > The functions in cirrus_vga_rop2.h are a bitmore complex. The safety > > checks *may* even be too strict in some cases but I haven't seen any > > aftefacts coming from too strong *range* checks (only the zero pitch > > check for which I sent a patch yesterday which I need to make a v2 for > > now :|). > > (Too strict in the pattern functions which clamp the source offsets > > with bitands, but I really don't feel like looking deep enough into > > this to make the checks even lighter :p) > > > >
Hi, > > The negative pitch means (I think) that "addr" points to the lower > > left corner of the rectangle. > > > > The second part guarantees that the last blitted byte fits (lower > > right corner). > > To which Gerd responded "upper left". In retrospect I don't understand > why we didn't discuss that question further, as it now seems that we > were both wrong -- "addr" stands for bottom right, in the negative pitch > case. /me looks at d3532a0db02296e687711b8cdc7791924efccea0 and I can't remember I wrote that code :-o And I can't remember the discussion either. The good thing is I probably looked more careful at the code because of that ... > Unfortunately, the original patch was meant to address the > then-embargoed CVE-2014-8106. Since we have a bug in that code (= a > security fix), this issue should have been reported privately as well, It has been reported privately first. I've actually suggested to send it to the public list without embargo, given that we are moving away from cirrus so this is less critical than it used to be two years ago. Cirrus isn't the default display adapter any more in qemu, since years, and management apps (virt-manager, ovirt, ...) are following. cheers, Gerd
Hi, > > I have read all the discuss, very long and useful, but I think I still > > need some > > time to get a full understand. So I think one of you can provide the > > formal patch to > > describe the issue in more detail. > > Your patch is almost correct. New version out for review. > Gerd, do you think you can raise this with the SRT? (I mean, if you > agree that this is a security issue.) Yes, it is. Getting a CVE number for it is wip. cheers, Gerd
On 01/25/17 08:18, Gerd Hoffmann wrote: > Hi, > >>> The negative pitch means (I think) that "addr" points to the lower >>> left corner of the rectangle. >>> >>> The second part guarantees that the last blitted byte fits (lower >>> right corner). >> >> To which Gerd responded "upper left". In retrospect I don't understand >> why we didn't discuss that question further, as it now seems that we >> were both wrong -- "addr" stands for bottom right, in the negative pitch >> case. > > /me looks at d3532a0db02296e687711b8cdc7791924efccea0 and I can't > remember I wrote that code :-o Haha, happens to me too :) > And I can't remember the discussion either. > > The good thing is I probably looked more careful at the code because of > that ... > >> Unfortunately, the original patch was meant to address the >> then-embargoed CVE-2014-8106. Since we have a bug in that code (= a >> security fix), this issue should have been reported privately as well, > > It has been reported privately first. I've actually suggested to send > it to the public list without embargo, given that we are moving away > from cirrus so this is less critical than it used to be two years ago. > Cirrus isn't the default display adapter any more in qemu, since years, > and management apps (virt-manager, ovirt, ...) are following. Ah, I see -- a CVE is justified, but an embargo: likely not. Makes sense. Thanks! Laszlo
diff --git a/hw/display/cirrus_vga.c b/hw/display/cirrus_vga.c index 379910d..7ddd289 100644 --- a/hw/display/cirrus_vga.c +++ b/hw/display/cirrus_vga.c @@ -277,7 +277,8 @@ static bool blit_region_is_unsafe(struct CirrusVGAState *s, } if (pitch < 0) { int64_t min = addr - + ((int64_t)s->cirrus_blt_height-1) * pitch; + + ((int64_t)s->cirrus_blt_height-1) * pitch + - s->cirrus_blt_width; int32_t max = addr + s->cirrus_blt_width; if (min < 0 || max > s->vga.vram_size) {