Fix OOB read in SgiRleDecode.c

* From Pillow 4.3.0->8.1.0
* CVE-2021-25293

Author: wiredfool

Tests/images/crash-465703f71a0f0094873a3e0e82c9f798161171b8.sgi

@@ -11,6 +11,13 @@
         "Tests/images/sgi_crash.bin",
         "Tests/images/crash-6b7f2244da6d0ae297ee0754a424213444e92778.sgi",
         "Tests/images/ossfuzz-5730089102868480.sgi",
+        "Tests/images/crash-754d9c7ec485ffb76a90eeaab191ef69a2a3a3cd.sgi",
+        "Tests/images/crash-465703f71a0f0094873a3e0e82c9f798161171b8.sgi",
+        "Tests/images/crash-64834657ee604b8797bf99eac6a194c124a9a8ba.sgi",
+        "Tests/images/crash-abcf1c97b8fe42a6c68f1fb0b978530c98d57ced.sgi",
+        "Tests/images/crash-b82e64d4f3f76d7465b6af535283029eda211259.sgi",
+        "Tests/images/crash-c1b2595b8b0b92cc5f38b6635e98e3a119ade807.sgi",
+        "Tests/images/crash-db8bfa78b19721225425530c5946217720d7df4e.sgi",
     ],
 )
 def test_crashes(test_file):

Tests/images/crash-64834657ee604b8797bf99eac6a194c124a9a8ba.sgi

@@ -25,12 +25,58 @@ read4B(UINT32 *dest, UINT8 *buf) {
     *dest = (UINT32)((buf[0] << 24) | (buf[1] << 16) | (buf[2] << 8) | buf[3]);
 }
 
+/*
+   SgiRleDecoding is done in a single channel row oriented set of RLE chunks.
+
+   * The file is arranged as
+     - SGI Header
+     - Rle Offset Table
+     - Rle Length Table
+     - Scanline Data
+
+   * Each RLE atom is c->bpc bytes wide (1 or 2)
+
+   * Each RLE Chunk is [specifier atom] [ 1 or n data atoms ]
+
+   * Copy Atoms are a byte with the high bit set, and the low 7 are
+     the number of bytes to copy from the source to the
+     destination. e.g.
+
+         CBBBBBBBB or 0CHLHLHLHLHLHL   (B=byte, H/L = Hi low bytes)
+
+   * Run atoms do not have the high bit set, and the low 7 bits are
+     the number of copies of the next atom to copy to the
+     destination. e.g.:
+
+         RB -> BBBBB or RHL -> HLHLHLHLHL
+
+   The upshot of this is, there is no way to determine the required
+   length of the input buffer from reloffset and rlelength without
+   going through the data at that scan line.
+
+   Furthermore, there's no requirement that individual scan lines
+   pointed to from the rleoffset table are in any sort of order or
+   used only once, or even disjoint. There's also no requirement that
+   all of the data in the scan line area of the image file be used
+
+ */
 static int
-expandrow(UINT8 *dest, UINT8 *src, int n, int z, int xsize) {
+expandrow(UINT8 *dest, UINT8 *src, int n, int z, int xsize, UINT8 *end_of_buffer) {
+    /*
+     * n here is the number of rlechunks
+     * z is the number of channels, for calculating the interleave
+     *   offset to go to RGBA style pixels
+     * xsize is the row width
+     * end_of_buffer is the address of the end of the input buffer
+     */
+
     UINT8 pixel, count;
     int x = 0;
 
     for (; n > 0; n--) {
+        if (src > end_of_buffer) {
+            return -1;
+        }
         pixel = *src++;
         if (n == 1 && pixel != 0) {
             return n;
@@ -44,12 +90,18 @@ expandrow(UINT8 *dest, UINT8 *src, int n, int z, int xsize) {
         }
         x += count;
         if (pixel & RLE_COPY_FLAG) {
+            if (src + count > end_of_buffer) {
+                return -1;
+            }
             while (count--) {
                 *dest = *src++;
                 dest += z;
             }
 
         } else {
+            if (src > end_of_buffer) {
+                return -1;
+            }
             pixel = *src++;
             while (count--) {
                 *dest = pixel;
@@ -61,12 +113,14 @@ expandrow(UINT8 *dest, UINT8 *src, int n, int z, int xsize) {
 }
 
 static int
-expandrow2(UINT8 *dest, const UINT8 *src, int n, int z, int xsize) {
+expandrow2(UINT8 *dest, const UINT8 *src, int n, int z, int xsize, UINT8 *end_of_buffer) {
     UINT8 pixel, count;
-
     int x = 0;
 
     for (; n > 0; n--) {
+        if (src + 1 > end_of_buffer) {
+            return -1;
+        }
         pixel = src[1];
         src += 2;
         if (n == 1 && pixel != 0) {
@@ -81,12 +135,18 @@ expandrow2(UINT8 *dest, const UINT8 *src, int n, int z, int xsize) {
         }
         x += count;
         if (pixel & RLE_COPY_FLAG) {
+            if (src + 2 * count > end_of_buffer) {
+                return -1;
+            }
             while (count--) {
                 memcpy(dest, src, 2);
                 src += 2;
                 dest += z * 2;
             }
         } else {
+            if (src + 2 > end_of_buffer) {
+                return -1;
+            }
             while (count--) {
                 memcpy(dest, src, 2);
                 dest += z * 2;
@@ -132,7 +192,11 @@ ImagingSgiRleDecode(Imaging im, ImagingCodecState state, UINT8 *buf, Py_ssize_t
         return -1;
     }
     _imaging_seek_pyFd(state->fd, SGI_HEADER_SIZE, SEEK_SET);
-    _imaging_read_pyFd(state->fd, (char *)ptr, c->bufsize);
+    if (_imaging_read_pyFd(state->fd, (char *)ptr, c->bufsize) != c->bufsize) {
+        state->errcode = IMAGING_CODEC_UNKNOWN;
+        return -1;
+    }
+
 
     /* decoder initialization */
     state->count = 0;
@@ -166,35 +230,37 @@ ImagingSgiRleDecode(Imaging im, ImagingCodecState state, UINT8 *buf, Py_ssize_t
         read4B(&c->lengthtab[c->tabindex], &ptr[c->bufindex]);
     }
 
-    state->count += c->tablen * sizeof(UINT32) * 2;
-
     /* read compressed rows */
     for (c->rowno = 0; c->rowno < im->ysize; c->rowno++, state->y += state->ystep) {
         for (c->channo = 0; c->channo < im->bands; c->channo++) {
             c->rleoffset = c->starttab[c->rowno + c->channo * im->ysize];
             c->rlelength = c->lengthtab[c->rowno + c->channo * im->ysize];
-            c->rleoffset -= SGI_HEADER_SIZE;
 
-            if (c->rleoffset + c->rlelength > c->bufsize) {
+            // Check for underflow of rleoffset-SGI_HEADER_SIZE
+            if (c->rleoffset < SGI_HEADER_SIZE) {
                 state->errcode = IMAGING_CODEC_OVERRUN;
                 goto sgi_finish_decode;
             }
 
+            c->rleoffset -= SGI_HEADER_SIZE;
+
             /* row decompression */
             if (c->bpc == 1) {
                 status = expandrow(
                     &state->buffer[c->channo],
                     &ptr[c->rleoffset],
                     c->rlelength,
                     im->bands,
-                    im->xsize);
+                    im->xsize,
+                    &ptr[c->bufsize-1]);
             } else {
                 status = expandrow2(
                     &state->buffer[c->channo * 2],
                     &ptr[c->rleoffset],
                     c->rlelength,
                     im->bands,
-                    im->xsize);
+                    im->xsize,
+                    &ptr[c->bufsize-1]);
             }
             if (status == -1) {
                 state->errcode = IMAGING_CODEC_OVERRUN;
@@ -203,15 +269,12 @@ ImagingSgiRleDecode(Imaging im, ImagingCodecState state, UINT8 *buf, Py_ssize_t
                 goto sgi_finish_decode;
             }
 
-            state->count += c->rlelength;
         }
 
         /* store decompressed data in image */
         state->shuffle((UINT8 *)im->image[state->y], state->buffer, im->xsize);
     }
 
-    c->bufsize++;
-
 sgi_finish_decode:;
 
     free(c->starttab);
@@ -221,5 +284,5 @@ sgi_finish_decode:;
         state->errcode = err;
         return -1;
     }
-    return state->count - c->bufsize;
+    return 0;
 }