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Message-ID: <010d5bdbbeabbb37f822e30a2451df7feedf051e.camel@intel.com>
Date: Thu, 7 Aug 2025 00:14:58 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "Reshetova, Elena" <elena.reshetova@...el.com>, "Hansen, Dave"
<dave.hansen@...el.com>
CC: "seanjc@...gle.com" <seanjc@...gle.com>, "mingo@...nel.org"
<mingo@...nel.org>, "Scarlata, Vincent R" <vincent.r.scarlata@...el.com>,
"x86@...nel.org" <x86@...nel.org>, "jarkko@...nel.org" <jarkko@...nel.org>,
"Annapurve, Vishal" <vannapurve@...gle.com>, "linux-kernel@...r.kernel.org"
<linux-kernel@...r.kernel.org>, "Mallick, Asit K" <asit.k.mallick@...el.com>,
"Aktas, Erdem" <erdemaktas@...gle.com>, "Cai, Chong" <chongc@...gle.com>,
"Bondarevska, Nataliia" <bondarn@...gle.com>, "linux-sgx@...r.kernel.org"
<linux-sgx@...r.kernel.org>, "Raynor, Scott" <scott.raynor@...el.com>
Subject: Re: [PATCH v11 4/5] x86/sgx: Implement ENCLS[EUPDATESVN]
On Wed, 2025-08-06 at 11:11 +0300, Elena Reshetova wrote:
> All running enclaves and cryptographic assets (such as internal SGX
> encryption keys) are assumed to be compromised whenever an SGX-related
> microcode update occurs. To mitigate this assumed compromise the new
> supervisor SGX instruction ENCLS[EUPDATESVN] can generate fresh
> cryptographic assets.
>
> Before executing EUPDATESVN, all SGX memory must be marked as unused.
> This requirement ensures that no potentially compromised enclave
> survives the update and allows the system to safely regenerate
> cryptographic assets.
>
> Add the method to perform ENCLS[EUPDATESVN]. However, until the
> follow up patch that wires calling sgx_update_svn() from
> sgx_inc_usage_count(), this code is not reachable.
Please check the text wrap.
>
> Signed-off-by: Elena Reshetova <elena.reshetova@...el.com>
> ---
> arch/x86/include/asm/sgx.h | 31 +++++++-------
> arch/x86/kernel/cpu/sgx/encls.h | 5 +++
> arch/x86/kernel/cpu/sgx/main.c | 73 +++++++++++++++++++++++++++++++++
> 3 files changed, 94 insertions(+), 15 deletions(-)
>
> diff --git a/arch/x86/include/asm/sgx.h b/arch/x86/include/asm/sgx.h
> index 2da5b3b117a1..0e13678f9cbd 100644
> --- a/arch/x86/include/asm/sgx.h
> +++ b/arch/x86/include/asm/sgx.h
> @@ -28,21 +28,22 @@
> #define SGX_CPUID_EPC_MASK GENMASK(3, 0)
>
> enum sgx_encls_function {
> - ECREATE = 0x00,
> - EADD = 0x01,
> - EINIT = 0x02,
> - EREMOVE = 0x03,
> - EDGBRD = 0x04,
> - EDGBWR = 0x05,
> - EEXTEND = 0x06,
> - ELDU = 0x08,
> - EBLOCK = 0x09,
> - EPA = 0x0A,
> - EWB = 0x0B,
> - ETRACK = 0x0C,
> - EAUG = 0x0D,
> - EMODPR = 0x0E,
> - EMODT = 0x0F,
> + ECREATE = 0x00,
> + EADD = 0x01,
> + EINIT = 0x02,
> + EREMOVE = 0x03,
> + EDGBRD = 0x04,
> + EDGBWR = 0x05,
> + EEXTEND = 0x06,
> + ELDU = 0x08,
> + EBLOCK = 0x09,
> + EPA = 0x0A,
> + EWB = 0x0B,
> + ETRACK = 0x0C,
> + EAUG = 0x0D,
> + EMODPR = 0x0E,
> + EMODT = 0x0F,
> + EUPDATESVN = 0x18,
> };
>
> /**
> diff --git a/arch/x86/kernel/cpu/sgx/encls.h b/arch/x86/kernel/cpu/sgx/encls.h
> index 99004b02e2ed..d9160c89a93d 100644
> --- a/arch/x86/kernel/cpu/sgx/encls.h
> +++ b/arch/x86/kernel/cpu/sgx/encls.h
> @@ -233,4 +233,9 @@ static inline int __eaug(struct sgx_pageinfo *pginfo, void *addr)
> return __encls_2(EAUG, pginfo, addr);
> }
>
> +/* Attempt to update CPUSVN at runtime. */
> +static inline int __eupdatesvn(void)
> +{
> + return __encls_ret_1(EUPDATESVN, "");
> +}
> #endif /* _X86_ENCLS_H */
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index 3a5cbd1c170e..d8c42524b590 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -16,6 +16,7 @@
> #include <linux/vmalloc.h>
> #include <asm/msr.h>
> #include <asm/sgx.h>
> +#include <asm/archrandom.h>
> #include "driver.h"
> #include "encl.h"
> #include "encls.h"
> @@ -917,6 +918,78 @@ int sgx_set_attribute(unsigned long *allowed_attributes,
> }
> EXPORT_SYMBOL_GPL(sgx_set_attribute);
>
> +/* Counter to count the active SGX users */
> +static int sgx_usage_count;
> +
> +/**
> + * sgx_update_svn() - Attempt to call ENCLS[EUPDATESVN].
Add a newline would make the comment more readable.
> + * This instruction attempts to update CPUSVN to the
> + * currently loaded microcode update SVN and generate new
> + * cryptographic assets.
Ditto here.
> + * Return:
> + * 0: Success or not supported
> + * -EAGAIN: Can be safely retried, failure is due to lack of
> + * entropy in RNG.
> + * -EIO: Unexpected error, retries are not advisable.
> + */
You may want to make the description of the return code vertically
aligned. And looking at the k-doc documentation, the format of the return
codes could be improved:
* Return:
* * %0: - Success or not supported
* * %-EAGAIN: - ...
* * %-EIO: - ...
Please see "Return values" part of:
https://docs.kernel.org/doc-guide/kernel-doc.html
Or you can switch to use normal comment. It's a static function anyway.
> +static int __maybe_unused sgx_update_svn(void)
> +{
> + int ret;
> +
> + /*
> + * If EUPDATESVN is not available, it is ok to
> + * silently skip it to comply with legacy behavior.
> + */
> + if (!cpu_feature_enabled(X86_FEATURE_SGX_EUPDATESVN))
> + return 0;
> +
> + /*
> + * EPC is guaranteed to be empty when there are no users.
> + * Ensure we are on our first user before proceeding further.
> + */
> + WARN(sgx_usage_count != 1, "Elevated usage count when calling EUPDATESVN\n");
I am not sure whether this is needed. Wouldn't the ENCLS_WARN() at the
end catch this case and the user is able to figure out what went wrong
from the error code?
Besides that, in _this_ patch, what prevents sgx_usage_count from being
concurrently updated is still unknown. It's kinda weird to just use it
here w/o seeing the actual mutex.
> +
> + for (int i = 0; i < RDRAND_RETRY_LOOPS; i++) {
> + ret = __eupdatesvn();
> +
> + /* Stop on success or unexpected errors: */
> + if (ret != SGX_INSUFFICIENT_ENTROPY)
> + break;
> + }
> +
> + switch (ret) {
> + /*
> + * SVN successfully updated.
> + * Let users know when the update was successful.
> + */
This is ugly. I would just put the comment inside the 'case'.
> + case 0:
> + pr_info("SVN updated successfully\n");
> + return 0;
> + /*
> + * SVN update failed since the current SVN is
> + * not newer than CPUSVN. This is the most
> + * common case and indicates no harm.
> + */
> + case SGX_NO_UPDATE:
> + return 0;
> + /*
> + * SVN update failed due to lack of entropy in DRNG.
> + * Indicate to userspace that it should retry.
> + */
> + case SGX_INSUFFICIENT_ENTROPY:
> + return -EAGAIN;
> + default:
> + break;
> + }
Ditto for all the comments above.
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