Messages in this thread Patch in this message | | | Date | Thu, 21 Apr 2022 16:20:54 -0700 (PDT) | From | Mat Martineau <> | Subject | Re: [PATCH mptcp-next] x86/pm: fix false positive kmemleak report in msr_build_context() |
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On Thu, 21 Apr 2022, Matthieu Baerts 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 [1] > - wait ~1 minute > - start a kmemleak scan > > It seems kmemleak has an issue with the array allocated in > msr_build_context() and assigned to a pointer in a static structure > (saved_context.saved_msrs->array): there is no leak then. > > It looks like this is a limitation from kmemleak but that's alright, > kmemleak_no_leak() can be used to avoid complaining about that. > > 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"). > > Depending on their CPUs, others have probably 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. But I > understand if someone says the origin of this issue is more on > kmemleak's side. What is unclear to me is why this issue was not seen by > other people and CIs. Maybe the kernel config [1]? > > [1] https://github.com/multipath-tcp/mptcp_net-next/files/8531660/kmemleak-cpu-sched-bisect.kconfig.txt >
Hi Matthieu -
It looks like the root cause here is alignment within the packed struct saved_context (from suspend_64.h). Kmemleak only searches for pointers that are aligned, but pahole shows that the saved_msrs struct member and all members after it in the structure are unaligned:
(gcc 11.2.1, x86_64)
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 odd offset values for the remainder of this struct vvv */
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__));
If I move 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 check wakeup_64.S and __save/__restore_processor_state() if the struct is modified - looks like it's the members before misc_enable that must be carefully placed.
So far I've only tried this on my local machine, I'll work on getting more thorough validation.
Looks like struct saved_context in suspend_32.h has similar odd alignment.
- Mat
> 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 > Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net> > --- > arch/x86/power/cpu.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c > index 3822666fb73d..1467c6d1a966 100644 > --- a/arch/x86/power/cpu.c > +++ b/arch/x86/power/cpu.c > @@ -14,6 +14,7 @@ > #include <linux/tboot.h> > #include <linux/dmi.h> > #include <linux/pgtable.h> > +#include <linux/kmemleak.h> > > #include <asm/proto.h> > #include <asm/mtrr.h> > @@ -413,6 +414,9 @@ static int msr_build_context(const u32 *msr_id, const int num) > return -ENOMEM; > } > > + /* The pointer is going to be stored in static struct (saved_context) */ > + kmemleak_not_leak(msr_array); > + > if (saved_msrs->array) { > /* > * Multiple callbacks can invoke this function, so copy any > -- > 2.34.1 > >
-- Mat Martineau Intel
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