x86/kprobes: Fix crash when probing CS CALL instructionsgrafted

When using eBPF to probe CS CALL instructions within a function,
a crash can be triggered.

The eBPF tool probes offset 257 of the __hrtimer_run_queues()
function:

<__hrtimer_run_queues+249>:  nopl   0x0(%rax,%rax,1)
<__hrtimer_run_queues+254>:  mov    %r14,%rdi
<__hrtimer_run_queues+257>:  cs call <__x86_indirect_thunk_r12>
<__hrtimer_run_queues+263>:  mov    %eax,%r12d
<__hrtimer_run_queues+266>:  xchg   %ax,%ax
<__hrtimer_run_queues+268>:  mov    %r13,%rdi

Which triggers this crash:

  BUG: unable to handle page fault for address: 00000000000f41c9
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  PGD 0 P4D 0
  Oops: 0002 [#1] SMP NOPTI
  CPU: 1 PID: 0 Comm: swapper/1 Kdump: loaded Tainted: P
  RIP: 0010:__hrtimer_run_queues+0x106/0x230

Note that __hrtimer_run_queues+0x106 is __hrtimer_run_queues+262, which is
at the 6th byte of the above CS CALL instruction. Since the CS CALL
instruction occupies 6 bytes, the exception occurred in the middle of that
call instruction.

The root cause is that when using eBPF tools to probe in the middle of a
function, a kprobe with INT3 is used as the underlying implementation.

During single-step emulation of the original CALL instruction,
int3_emulate_call() assumes that the probed CALL instruction is 5 bytes
long. However, the actual CS-prefixed CALL instruction occupies 6 bytes,
so it constructs an incorrect exception return address. When the CPU
returns from the kprobe handler, the next instruction to be executed is at
the address of the last byte of that CS CALL instruction. Coincidentally,
starting from that address, the CPU fetches and decodes a completely
different instruction, which ultimately triggers a kernel crash.

Fix the issue by using the actual instruction length obtained from
the instruction decoder when constructing the exception return
address, rather than relying on the hardcoded CALL_INSN_SIZE macro.

[ mingo: Refined the changelog ]

Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step")
Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Jinke Han <jinkehan@didiglobal.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://patch.msgid.link/20260908073742.GA10517@didi-ThinkCentre-M920t-N000

1 files changed, 187 insertions, 0 deletions

diff --git a/Documentation/crypto/api-samples.rst b/Documentation/crypto/api-samples.rst
new file mode 100644
index 000000000..388bb7d7a
--- /dev/null
+++ b/Documentation/crypto/api-samples.rst
@@ -0,0 +1,187 @@
+Code Examples
+=============
+
+Code Example For Symmetric Key Cipher Operation
+-----------------------------------------------
+
+This code encrypts some data with AES-256-XTS.  For sake of example,
+all inputs are random bytes, the encryption is done in-place, and it's
+assumed the code is running in a context where it can sleep.
+
+::
+
+    static int test_skcipher(void)
+    {
+            struct crypto_skcipher *tfm = NULL;
+            struct skcipher_request *req = NULL;
+            u8 *data = NULL;
+            const size_t datasize = 512; /* data size in bytes */
+            struct scatterlist sg;
+            DECLARE_CRYPTO_WAIT(wait);
+            u8 iv[16];  /* AES-256-XTS takes a 16-byte IV */
+            u8 key[64]; /* AES-256-XTS takes a 64-byte key */
+            int err;
+
+            /*
+             * Allocate a tfm (a transformation object) and set the key.
+             *
+             * In real-world use, a tfm and key are typically used for many
+             * encryption/decryption operations.  But in this example, we'll just do a
+             * single encryption operation with it (which is not very efficient).
+             */
+
+            tfm = crypto_alloc_skcipher("xts(aes)", 0, 0);
+            if (IS_ERR(tfm)) {
+                    pr_err("Error allocating xts(aes) handle: %ld\n", PTR_ERR(tfm));
+                    return PTR_ERR(tfm);
+            }
+
+            get_random_bytes(key, sizeof(key));
+            err = crypto_skcipher_setkey(tfm, key, sizeof(key));
+            if (err) {
+                    pr_err("Error setting key: %d\n", err);
+                    goto out;
+            }
+
+            /* Allocate a request object */
+            req = skcipher_request_alloc(tfm, GFP_KERNEL);
+            if (!req) {
+                    err = -ENOMEM;
+                    goto out;
+            }
+
+            /* Prepare the input data */
+            data = kmalloc(datasize, GFP_KERNEL);
+            if (!data) {
+                    err = -ENOMEM;
+                    goto out;
+            }
+            get_random_bytes(data, datasize);
+
+            /* Initialize the IV */
+            get_random_bytes(iv, sizeof(iv));
+
+            /*
+             * Encrypt the data in-place.
+             *
+             * For simplicity, in this example we wait for the request to complete
+             * before proceeding, even if the underlying implementation is asynchronous.
+             *
+             * To decrypt instead of encrypt, just change crypto_skcipher_encrypt() to
+             * crypto_skcipher_decrypt().
+             */
+            sg_init_one(&sg, data, datasize);
+            skcipher_request_set_callback(req, CRYPTO_TFM_REQ_MAY_BACKLOG |
+                                               CRYPTO_TFM_REQ_MAY_SLEEP,
+                                          crypto_req_done, &wait);
+            skcipher_request_set_crypt(req, &sg, &sg, datasize, iv);
+            err = crypto_wait_req(crypto_skcipher_encrypt(req), &wait);
+            if (err) {
+                    pr_err("Error encrypting data: %d\n", err);
+                    goto out;
+            }
+
+            pr_debug("Encryption was successful\n");
+    out:
+            crypto_free_skcipher(tfm);
+            skcipher_request_free(req);
+            kfree(data);
+            return err;
+    }
+
+
+Code Example For Use of Operational State Memory With SHASH
+-----------------------------------------------------------
+
+::
+
+
+    struct sdesc {
+        struct shash_desc shash;
+        char ctx[];
+    };
+
+    static struct sdesc *init_sdesc(struct crypto_shash *alg)
+    {
+        struct sdesc *sdesc;
+        int size;
+
+        size = sizeof(struct shash_desc) + crypto_shash_descsize(alg);
+        sdesc = kmalloc(size, GFP_KERNEL);
+        if (!sdesc)
+            return ERR_PTR(-ENOMEM);
+        sdesc->shash.tfm = alg;
+        return sdesc;
+    }
+
+    static int calc_hash(struct crypto_shash *alg,
+                 const unsigned char *data, unsigned int datalen,
+                 unsigned char *digest)
+    {
+        struct sdesc *sdesc;
+        int ret;
+
+        sdesc = init_sdesc(alg);
+        if (IS_ERR(sdesc)) {
+            pr_info("can't alloc sdesc\n");
+            return PTR_ERR(sdesc);
+        }
+
+        ret = crypto_shash_digest(&sdesc->shash, data, datalen, digest);
+        kfree(sdesc);
+        return ret;
+    }
+
+    static int test_hash(const unsigned char *data, unsigned int datalen,
+                 unsigned char *digest)
+    {
+        struct crypto_shash *alg;
+        char *hash_alg_name = "sha1-padlock-nano";
+        int ret;
+
+        alg = crypto_alloc_shash(hash_alg_name, 0, 0);
+        if (IS_ERR(alg)) {
+                pr_info("can't alloc alg %s\n", hash_alg_name);
+                return PTR_ERR(alg);
+        }
+        ret = calc_hash(alg, data, datalen, digest);
+        crypto_free_shash(alg);
+        return ret;
+    }
+
+
+Code Example For Random Number Generator Usage
+----------------------------------------------
+
+::
+
+
+    static int get_random_numbers(u8 *buf, unsigned int len)
+    {
+        struct crypto_rng *rng = NULL;
+        char *drbg = "stdrng";
+        int ret;
+
+        if (!buf || !len) {
+            pr_debug("No output buffer provided\n");
+            return -EINVAL;
+        }
+
+        rng = crypto_alloc_rng(drbg, 0, 0);
+        if (IS_ERR(rng)) {
+            pr_debug("could not allocate RNG handle for %s\n", drbg);
+            return PTR_ERR(rng);
+        }
+
+        ret = crypto_rng_get_bytes(rng, buf, len);
+        if (ret < 0)
+            pr_debug("generation of random numbers failed\n");
+        else if (ret == 0)
+            pr_debug("RNG returned no data");
+        else
+            pr_debug("RNG returned %d bytes of data\n", ret);
+
+    out:
+        crypto_free_rng(rng);
+        return ret;
+    }