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Tue, 18 Mar 2025 11:36:07 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id MD0CAqda2WeDTQAAD6G6ig (envelope-from ); Tue, 18 Mar 2025 11:36:07 +0000 From: Nikolay Borisov To: dave.hansen@linux.intel.com Cc: kirill.shutemov@linux.intel.com, linux-coco@lists.linux.dev, x86@kernel.org, linux-kernel@vger.kernel.org, vannapurve@google.com, Nikolay Borisov Subject: [RFC PATCH] /dev/mem: Disable /dev/mem under TDX guest Date: Tue, 18 Mar 2025 13:36:04 +0200 Message-ID: <20250318113604.297726-1-nik.borisov@suse.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Spam-Level: X-Spamd-Result: default: False [-2.80 / 50.00]; BAYES_HAM(-3.00)[100.00%]; MID_CONTAINS_FROM(1.00)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_MISSING_CHARSET(0.50)[]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; ARC_NA(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; MIME_TRACE(0.00)[0:+]; FUZZY_BLOCKED(0.00)[rspamd.com]; DKIM_SIGNED(0.00)[suse.com:s=susede1]; RCVD_TLS_ALL(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; RCPT_COUNT_SEVEN(0.00)[7]; RCVD_COUNT_TWO(0.00)[2]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.com:email,suse.com:mid,imap1.dmz-prg2.suse.org:helo] X-Spam-Score: -2.80 X-Spam-Flag: NO Content-Type: text/plain; charset="utf-8" If a piece of memory is read from /dev/mem that falls outside of the System Ram region i.e bios data region the kernel creates a shared mapping via xlate_dev_mem_ptr() (this behavior was introduced by 9aa6ea69852c ("x86/tdx: Make pages shared in ioremap()"). This results in a region having both a shared and a private mapping. Subsequent accesses to this region via the private mapping induce a SEPT violation and a crash of the VMM. In this particular case the scenario was a userspace process reading something from the bios data area at address 0x497 which creates a shared mapping, and a followup reboot accessing __va(0x472) which access pfn 0 via the private mapping causing mayhem. Fix this by simply forbidding access to /dev/mem when running as an TDX guest. Signed-off-by: Nikolay Borisov Tested-by: Nikolay Borisov --- Sending this now to hopefully spur up discussion as to how to handle the de= scribed scenario. This was hit on the GCP cloud and was causing their hypervisor to= crash. I guess the most pressing question is what will be the most sensible approa= ch to eliminate such situations happening in the future: 1. Should we forbid getting a descriptor to /dev/mem (this patch) 2. Skip creating /dev/mem altogether3 3. Possibly tinker with internals of ioremap to ensure that no memory which= is backed by kvm memslots is remapped as shared. 4. Eliminate the access to 0x472 from the x86 reboot path, after all we don= 't really have a proper bios at that address. 5. Something else ? arch/x86/coco/tdx/tdx.c | 4 ++++ drivers/char/mem.c | 3 +++ include/linux/security.h | 6 ++++++ 3 files changed, 13 insertions(+) diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c index 32809a06dab4..615e8a300fc7 100644 --- a/arch/x86/coco/tdx/tdx.c +++ b/arch/x86/coco/tdx/tdx.c @@ -40,6 +40,8 @@ static atomic_long_t nr_shared; +bool devmem_disabled =3D false; + /* Called from __tdx_hypercall() for unrecoverable failure */ noinstr void __noreturn __tdx_hypercall_failed(void) { @@ -1063,6 +1065,8 @@ void __init tdx_early_init(void) setup_force_cpu_cap(X86_FEATURE_TDX_GUEST); + devmem_disabled =3D true; + /* TSC is the only reliable clock in TDX guest */ setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE); diff --git a/drivers/char/mem.c b/drivers/char/mem.c index 169eed162a7f..8778d46216f2 100644 --- a/drivers/char/mem.c +++ b/drivers/char/mem.c @@ -616,6 +616,9 @@ static int open_port(struct inode *inode, struct file *= filp) if (iminor(inode) !=3D DEVMEM_MINOR) return 0; + if (devmem_disabled) + return -EINVAL; + /* * Use a unified address space to have a single point to manage * revocations when drivers want to take over a /dev/mem mapped diff --git a/include/linux/security.h b/include/linux/security.h index 980b6c207cad..1757f683a09d 100644 --- a/include/linux/security.h +++ b/include/linux/security.h @@ -265,6 +265,12 @@ struct request_sock; #define LSM_UNSAFE_PTRACE 2 #define LSM_UNSAFE_NO_NEW_PRIVS 4 +#ifdef CONFIG_INTEL_TDX_GUEST +extern bool devmem_disabled; +#else +#define devmem_disabled 0 +#endif + #ifdef CONFIG_MMU extern int mmap_min_addr_handler(const struct ctl_table *table, int write, void *buffer, size_t *lenp, loff_t *ppos); -- 2.43.0