dmaengine: fix use-after-free in dma_chan_put() and dma_release_channel()grafted

When dma_device_put() drops the last reference on chan->device->ref,
dma_device_release() runs and may free the dma_device along with its
channels.

dma_chan_put() then still reads chan->device->owner via
dma_chan_to_owner() for the trailing module_put(). KASAN catches it:

	slab-use-after-free in dma_chan_put+0x3e6/0x4c0
	Read of size 8 by task insmod/6319
	Freed by task 6319:
	  kfree+0x225/0x470
	  dma_chan_put+0x395/0x4c0
	  dmaengine_put+0xf8/0x160

Cache the module owner in dma_chan_put() before the put so the trailing
module_put() does not need chan->device.

Fixes: 8ad342a86359 ("dmaengine: Add reference counting to dma_device struct")
Suggested-by: Sashiko <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/20260518-dmaengine-kref-fix-v1-1-4d6125048fb7@amd.com
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Shivank Garg <shivankg@amd.com>
Link: https://patch.msgid.link/20260822-dmaengine-kref-fix-v5-3-d4a4ee47d927@amd.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

1 files changed, 56 insertions, 0 deletions

diff --git a/Documentation/fb/cmap_xfbdev.rst b/Documentation/fb/cmap_xfbdev.rst
new file mode 100644
index 000000000..5db5e9787
--- /dev/null
+++ b/Documentation/fb/cmap_xfbdev.rst
@@ -0,0 +1,56 @@
+==========================
+Understanding fbdev's cmap
+==========================
+
+These notes explain how X's dix layer uses fbdev's cmap structures.
+
+-  example of relevant structures in fbdev as used for a 3-bit grayscale cmap::
+
+    struct fb_var_screeninfo {
+	    .bits_per_pixel = 8,
+	    .grayscale      = 1,
+	    .red =          { 4, 3, 0 },
+	    .green =        { 0, 0, 0 },
+	    .blue =         { 0, 0, 0 },
+    }
+    struct fb_fix_screeninfo {
+	    .visual =       FB_VISUAL_STATIC_PSEUDOCOLOR,
+    }
+    for (i = 0; i < 8; i++)
+	info->cmap.red[i] = (((2*i)+1)*(0xFFFF))/16;
+    memcpy(info->cmap.green, info->cmap.red, sizeof(u16)*8);
+    memcpy(info->cmap.blue, info->cmap.red, sizeof(u16)*8);
+
+-  X11 apps do something like the following when trying to use grayscale::
+
+    for (i=0; i < 8; i++) {
+	char colorspec[64];
+	memset(colorspec,0,64);
+	sprintf(colorspec, "rgb:%x/%x/%x", i*36,i*36,i*36);
+	if (!XParseColor(outputDisplay, testColormap, colorspec, &wantedColor))
+		printf("Can't get color %s\n",colorspec);
+	XAllocColor(outputDisplay, testColormap, &wantedColor);
+	grays[i] = wantedColor;
+    }
+
+There's also named equivalents like gray1..x provided you have an rgb.txt.
+
+Somewhere in X's callchain, this results in a call to X code that handles the
+colormap. For example, Xfbdev hits the following:
+
+xc-011010/programs/Xserver/dix/colormap.c::
+
+  FindBestPixel(pentFirst, size, prgb, channel)
+
+  dr = (long) pent->co.local.red - prgb->red;
+  dg = (long) pent->co.local.green - prgb->green;
+  db = (long) pent->co.local.blue - prgb->blue;
+  sq = dr * dr;
+  UnsignedToBigNum (sq, &sum);
+  BigNumAdd (&sum, &temp, &sum);
+
+co.local.red are entries that were brought in through FBIOGETCMAP which come
+directly from the info->cmap.red that was listed above. The prgb is the rgb
+that the app wants to match to. The above code is doing what looks like a least
+squares matching function. That's why the cmap entries can't be set to the left
+hand side boundaries of a color range.