CVE-2022-49287

In the Linux kernel, the following vulnerability has been resolved:

tpm: fix reference counting for struct tpm_chip

The following sequence of operations results in a refcount warning:

1. Open device /dev/tpmrm.
2. Remove module tpm_tis_spi.
3. Write a TPM command to the file descriptor opened at step 1.

————[ cut here ]———— WARNING: CPU: 3 PID: 1161 at lib/refcount.c:25 kobject_get+0xa0/0xa4 refcount_t: addition on 0; use-after-free. Modules linked in: tpm_tis_spi tpm_tis_core tpm mdio_bcm_unimac brcmfmac sha256_generic libsha256 sha256_arm hci_uart btbcm bluetooth cfg80211 vc4 brcmutil ecdh_generic ecc snd_soc_core crc32_arm_ce libaes raspberrypi_hwmon ac97_bus snd_pcm_dmaengine bcm2711_thermal snd_pcm snd_timer genet snd phy_generic soundcore [last unloaded: spi_bcm2835] CPU: 3 PID: 1161 Comm: hold_open Not tainted 5.10.0ls-main-dirty #2 Hardware name: BCM2711 [] (unwind_backtrace) from [] (show_stack+0x10/0x14) [] (show_stack) from [] (dump_stack+0xc4/0xd8) [] (dump_stack) from [] (__warn+0x104/0x108) [] (__warn) from [] (warn_slowpath_fmt+0x74/0xb8) [] (warn_slowpath_fmt) from [] (kobject_get+0xa0/0xa4) [] (kobject_get) from [] (tpm_try_get_ops+0x14/0x54 [tpm]) [] (tpm_try_get_ops [tpm]) from [] (tpm_common_write+0x38/0x60 [tpm]) [] (tpm_common_write [tpm]) from [] (vfs_write+0xc4/0x3c0) [] (vfs_write) from [] (ksys_write+0x58/0xcc) [] (ksys_write) from [] (ret_fast_syscall+0x0/0x4c) Exception stack(0xc226bfa8 to 0xc226bff0) bfa0: 00000000 000105b4 00000003 beafe664 00000014 00000000 bfc0: 00000000 000105b4 000103f8 00000004 00000000 00000000 b6f9c000 beafe684 bfe0: 0000006c beafe648 0001056c b6eb6944 —[ end trace d4b8409def9b8b1f ]—

The reason for this warning is the attempt to get the chip->dev reference in tpm_common_write() although the reference counter is already zero.

Since commit 8979b02aaf1d (tpm: Fix reference count to main device) the extra reference used to prevent a premature zero counter is never taken, because the required TPM_CHIP_FLAG_TPM2 flag is never set.

Fix this by moving the TPM 2 character device handling from tpm_chip_alloc() to tpm_add_char_device() which is called at a later point in time when the flag has been set in case of TPM2.

Commit fdc915f7f719 (tpm: expose spaces via a device link /dev/tpmrm) already introduced function tpm_devs_release() to release the extra reference but did not implement the required put on chip->devs that results in the call of this function.

Fix this by putting chip->devs in tpm_chip_unregister().

Finally move the new implementation for the TPM 2 handling into a new function to avoid multiple checks for the TPM_CHIP_FLAG_TPM2 flag in the good case and error cases.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/290e05f346d1829e849662c97e42d5ad984f5258
• https://git.kernel.org/stable/c/2f928c0d5c02dbab49e8c19d98725c822f6fc409
• https://git.kernel.org/stable/c/473a66f99cb8173c14138c5a5c69bfad04e8f9ac
• https://git.kernel.org/stable/c/662893b4f6bd466ff9e1cd454c44c26d32d554fe
• https://git.kernel.org/stable/c/6e7baf84149fb43950631415de231b3a41915aa3
• https://git.kernel.org/stable/c/7e0438f83dc769465ee663bb5dcf8cc154940712
• https://git.kernel.org/stable/c/a27ed2f3695baf15f9b34d2d7a1f9fc105539a81
• https://git.kernel.org/stable/c/cb64bd038beacb4331fe464a36c8b5481e8f51e2