gh-119451: Fix a potential denial of service in http.client (GH-119454)

Reading the whole body of the HTTP response could cause OOM if
the Content-Length value is too large even if the server does not send
a large amount of data. Now the HTTP client reads large data by chunks,
therefore the amount of consumed memory is proportional to the amount
of sent data.
(cherry picked from commit 5a4c4a0)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Author: serhiy-storchaka

Lib/http/client.py

@@ -111,6 +111,11 @@
 _MAXLINE = 65536
 _MAXHEADERS = 100
 
+# Data larger than this will be read in chunks, to prevent extreme
+# overallocation.
+_MIN_READ_BUF_SIZE = 1 << 20
+
+
 # Header name/value ABNF (http://tools.ietf.org/html/rfc7230#section-3.2)
 #
 # VCHAR          = %x21-7E
@@ -628,10 +633,25 @@ def _safe_read(self, amt):
         reading. If the bytes are truly not available (due to EOF), then the
         IncompleteRead exception can be used to detect the problem.
         """
-        data = self.fp.read(amt)
-        if len(data) < amt:
-            raise IncompleteRead(data, amt-len(data))
-        return data
+        cursize = min(amt, _MIN_READ_BUF_SIZE)
+        data = self.fp.read(cursize)
+        if len(data) >= amt:
+            return data
+        if len(data) < cursize:
+            raise IncompleteRead(data, amt - len(data))
+
+        data = io.BytesIO(data)
+        data.seek(0, 2)
+        while True:
+            # This is a geometric increase in read size (never more than
+            # doubling out the current length of data per loop iteration).
+            delta = min(cursize, amt - cursize)
+            data.write(self.fp.read(delta))
+            if data.tell() >= amt:
+                return data.getvalue()
+            cursize += delta
+            if data.tell() < cursize:
+                raise IncompleteRead(data.getvalue(), amt - data.tell())
 
     def _safe_readinto(self, b):
         """Same as _safe_read, but for reading into a buffer."""

Lib/test/test_httplib.py

@@ -1226,6 +1226,72 @@ def run_server():
         thread.join()
         self.assertEqual(result, b"proxied data\n")
 
+    def test_large_content_length(self):
+        serv = socket.create_server((HOST, 0))
+        self.addCleanup(serv.close)
+
+        def run_server():
+            [conn, address] = serv.accept()
+            with conn:
+                while conn.recv(1024):
+                    conn.sendall(
+                        b"HTTP/1.1 200 Ok\r\n"
+                        b"Content-Length: %d\r\n"
+                        b"\r\n" % size)
+                    conn.sendall(b'A' * (size//3))
+                    conn.sendall(b'B' * (size - size//3))
+
+        thread = threading.Thread(target=run_server)
+        thread.start()
+        self.addCleanup(thread.join, 1.0)
+
+        conn = client.HTTPConnection(*serv.getsockname())
+        try:
+            for w in range(15, 27):
+                size = 1 << w
+                conn.request("GET", "/")
+                with conn.getresponse() as response:
+                    self.assertEqual(len(response.read()), size)
+        finally:
+            conn.close()
+            thread.join(1.0)
+
+    def test_large_content_length_truncated(self):
+        serv = socket.create_server((HOST, 0))
+        self.addCleanup(serv.close)
+
+        def run_server():
+            while True:
+                [conn, address] = serv.accept()
+                with conn:
+                    conn.recv(1024)
+                    if not size:
+                        break
+                    conn.sendall(
+                        b"HTTP/1.1 200 Ok\r\n"
+                        b"Content-Length: %d\r\n"
+                        b"\r\n"
+                        b"Text" % size)
+
+        thread = threading.Thread(target=run_server)
+        thread.start()
+        self.addCleanup(thread.join, 1.0)
+
+        conn = client.HTTPConnection(*serv.getsockname())
+        try:
+            for w in range(18, 65):
+                size = 1 << w
+                conn.request("GET", "/")
+                with conn.getresponse() as response:
+                    self.assertRaises(client.IncompleteRead, response.read)
+                conn.close()
+        finally:
+            conn.close()
+            size = 0
+            conn.request("GET", "/")
+            conn.close()
+            thread.join(1.0)
+
     def test_putrequest_override_domain_validation(self):
         """
         It should be possible to override the default validation

Misc/NEWS.d/next/Security/2024-05-23-11-47-48.gh-issue-119451.qkJe9-.rst

@@ -0,0 +1,5 @@
+Fix a potential memory denial of service in the :mod:`http.client` module.
+When connecting to a malicious server, it could cause
+an arbitrary amount of memory to be allocated.
+This could have led to symptoms including a :exc:`MemoryError`, swapping, out
+of memory (OOM) killed processes or containers, or even system crashes.