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Date: Wed, 27 Apr 2022 18:33:22 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Matthieu Baerts <matthieu.baerts@...sares.net>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
Pavel Machek <pavel@....cz>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"the arch/x86 maintainers" <x86@...nel.org>,
"H. Peter Anvin" <hpa@...or.com>, Chen Yu <yu.c.chen@...el.com>,
Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
Catalin Marinas <catalin.marinas@....com>,
Linux Memory Management List <linux-mm@...ck.org>,
Mat Martineau <mathew.j.martineau@...ux.intel.com>,
"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
Ingo Molnar <mingo@...nel.org>,
Linux PM <linux-pm@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] x86/pm: fix false positive kmemleak report in msr_build_context()
On Tue, Apr 26, 2022 at 10:22 PM Matthieu Baerts
<matthieu.baerts@...sares.net> wrote:
>
> Since commit e2a1256b17b1 ("x86/speculation: Restore speculation related MSRs during S3 resume"),
> kmemleak reports this issue:
>
> unreferenced object 0xffff888009cedc00 (size 256):
> comm "swapper/0", pid 1, jiffies 4294693823 (age 73.764s)
> hex dump (first 32 bytes):
> 00 00 00 00 00 00 00 00 48 00 00 00 00 00 00 00 ........H.......
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
> backtrace:
> msr_build_context (include/linux/slab.h:621)
> pm_check_save_msr (arch/x86/power/cpu.c:520)
> do_one_initcall (init/main.c:1298)
> kernel_init_freeable (init/main.c:1370)
> kernel_init (init/main.c:1504)
> ret_from_fork (arch/x86/entry/entry_64.S:304)
>
> It is easy to reproduce it on my side:
>
> - boot the VM with a debug kernel config (see the 'Closes:' tag)
> - wait ~1 minute
> - start a kmemleak scan
>
> It seems kmemleak has an issue with the array allocated in
> msr_build_context(). This array is assigned to a pointer in a static
> structure (saved_context.saved_msrs->array): there is no leak then.
>
> A simple fix for this issue would be to use kmemleak_no_leak() but Mat
> noticed that the root cause here is alignment within the packed 'struct
> saved_context' (from suspend_64.h). Kmemleak only searches for pointers
> that are aligned (see how pointers are scanned in kmemleak.c), but
> pahole shows that the saved_msrs struct member and all members after it
> in the structure are unaligned:
>
> struct saved_context {
> struct pt_regs regs; /* 0 168 */
> /* --- cacheline 2 boundary (128 bytes) was 40 bytes ago --- */
> u16 ds; /* 168 2 */
> u16 es; /* 170 2 */
> u16 fs; /* 172 2 */
> u16 gs; /* 174 2 */
> long unsigned int kernelmode_gs_base; /* 176 8 */
> long unsigned int usermode_gs_base; /* 184 8 */
> /* --- cacheline 3 boundary (192 bytes) --- */
> long unsigned int fs_base; /* 192 8 */
> long unsigned int cr0; /* 200 8 */
> long unsigned int cr2; /* 208 8 */
> long unsigned int cr3; /* 216 8 */
> long unsigned int cr4; /* 224 8 */
> u64 misc_enable; /* 232 8 */
> bool misc_enable_saved; /* 240 1 */
>
> /* Note below odd offset values for the remainder of this struct */
>
> struct saved_msrs saved_msrs; /* 241 16 */
> /* --- cacheline 4 boundary (256 bytes) was 1 bytes ago --- */
> long unsigned int efer; /* 257 8 */
> u16 gdt_pad; /* 265 2 */
> struct desc_ptr gdt_desc; /* 267 10 */
> u16 idt_pad; /* 277 2 */
> struct desc_ptr idt; /* 279 10 */
> u16 ldt; /* 289 2 */
> u16 tss; /* 291 2 */
> long unsigned int tr; /* 293 8 */
> long unsigned int safety; /* 301 8 */
> long unsigned int return_address; /* 309 8 */
>
> /* size: 317, cachelines: 5, members: 25 */
> /* last cacheline: 61 bytes */
> } __attribute__((__packed__));
>
> By moving 'misc_enable_saved' to the end of the struct declaration,
> 'saved_msrs' fits in before the cacheline 4 boundary and the kmemleak
> warning goes away.
>
> The comment above the 'saved_context' declaration says to fix
> wakeup_64.S file and __save/__restore_processor_state() if the struct is
> modified: it looks like all the accesses in wakeup_64.S are done through
> offsets which are computed at build-time. This comment has been updated
> accordingly.
>
> At the end, the false positive kmemleak report is due to a limitation
> from kmemleak but that's always good to avoid unaligned member for
> optimisation purposes.
>
> Please note that it looks like this issue is not new, e.g.
>
> https://lore.kernel.org/all/9f1bb619-c4ee-21c4-a251-870bd4db04fa@lwfinger.net/
> https://lore.kernel.org/all/94e48fcd-1dbd-ebd2-4c91-f39941735909@molgen.mpg.de/
>
> But on my side, msr_build_context() is only used since:
>
> commit e2a1256b17b1 ("x86/speculation: Restore speculation related MSRs during S3 resume").
>
> Others probably have the same issue since:
>
> commit 7a9c2dd08ead ("x86/pm: Introduce quirk framework to save/restore extra MSR registers around suspend/resume"),
>
> Hence the 'Fixes' tag here below to help with the backports.
>
> Fixes: 7a9c2dd08ead ("x86/pm: Introduce quirk framework to save/restore extra MSR registers around suspend/resume")
> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/268
> Suggested-by: Mat Martineau <mathew.j.martineau@...ux.intel.com>
> Signed-off-by: Matthieu Baerts <matthieu.baerts@...sares.net>
Still good IMV.
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>
> Notes:
> v3:
> - update the comment above 'saved_context' structure (Borislav)
> v2:
> - update 'saved_context' structure instead of using kmemleak_no_leak() (Mat)
>
> arch/x86/include/asm/suspend_32.h | 2 +-
> arch/x86/include/asm/suspend_64.h | 12 ++++++++----
> 2 files changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/include/asm/suspend_32.h b/arch/x86/include/asm/suspend_32.h
> index 7b132d0312eb..a800abb1a992 100644
> --- a/arch/x86/include/asm/suspend_32.h
> +++ b/arch/x86/include/asm/suspend_32.h
> @@ -19,7 +19,6 @@ struct saved_context {
> u16 gs;
> unsigned long cr0, cr2, cr3, cr4;
> u64 misc_enable;
> - bool misc_enable_saved;
> struct saved_msrs saved_msrs;
> struct desc_ptr gdt_desc;
> struct desc_ptr idt;
> @@ -28,6 +27,7 @@ struct saved_context {
> unsigned long tr;
> unsigned long safety;
> unsigned long return_address;
> + bool misc_enable_saved;
> } __attribute__((packed));
>
> /* routines for saving/restoring kernel state */
> diff --git a/arch/x86/include/asm/suspend_64.h b/arch/x86/include/asm/suspend_64.h
> index 35bb35d28733..0dc400fae1b2 100644
> --- a/arch/x86/include/asm/suspend_64.h
> +++ b/arch/x86/include/asm/suspend_64.h
> @@ -14,9 +14,13 @@
> * Image of the saved processor state, used by the low level ACPI suspend to
> * RAM code and by the low level hibernation code.
> *
> - * If you modify it, fix arch/x86/kernel/acpi/wakeup_64.S and make sure that
> - * __save/__restore_processor_state(), defined in arch/x86/kernel/suspend_64.c,
> - * still work as required.
> + * If you modify it, check how it is used in arch/x86/kernel/acpi/wakeup_64.S
> + * and make sure that __save/__restore_processor_state(), defined in
> + * arch/x86/power/cpu.c, still work as required.
> + *
> + * Because the structure is packed, make sure to avoid unaligned members. For
> + * optimisations purposes but also because tools like Kmemleak only search for
> + * pointers that are aligned.
> */
> struct saved_context {
> struct pt_regs regs;
> @@ -36,7 +40,6 @@ struct saved_context {
>
> unsigned long cr0, cr2, cr3, cr4;
> u64 misc_enable;
> - bool misc_enable_saved;
> struct saved_msrs saved_msrs;
> unsigned long efer;
> u16 gdt_pad; /* Unused */
> @@ -48,6 +51,7 @@ struct saved_context {
> unsigned long tr;
> unsigned long safety;
> unsigned long return_address;
> + bool misc_enable_saved;
> } __attribute__((packed));
>
> #define loaddebug(thread,register) \
> --
> 2.34.1
>
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